maternal–fetal blood incompatibility and the risk of schizophrenia in offspring

www.elsevier.com/locate/schres

Schizophrenia Research

Maternal–fetal blood incompatibility and the risk of schizophrenia

in offspring

Beverly J. Insel *, Alan S. Brown, Michaeline A. Bresnahan,

Catherine A. Schaefer, Ezra S. Susser

Mailman School of Public Health of Columbia University Epidemiology, 722 W. 168 St., New York 10032, United States

Received 18 November 2004; received in revised form 26 May 2005; accepted 6 June 2005

Available online 11 July 2005

Abstract

Objective: Predicated on a maternal immune response to paternally inherited foreign fetal blood antigens, we hypothesized that

maternal–fetal blood incompatibility increases susceptibility to schizophrenia in the offspring. The relation between schizo-

phrenia and maternal–fetal blood incompatibility, arising from the D antigen of the Rhesus (Rh) and the ABO blood group

antigens, was examined in a cohort of live-births.

Method: The data were drawn from the Prenatal Determinants of Schizophrenia Study, a cohort of births occurring between

1959 and 1967 to women enrolled in a Kaiser Permanente Plan-Northern California Region (KP). Adult offspring belonging to

the KP from 1981 to 1997 were followed for the incidence of schizophrenia spectrum disorder (SSD). Cox proportional hazards

regression was the primary analytic technique.

Results: Among second and later born offspring, the adjusted incidence rate ratio (RRadj) of SSD was 1.80 (95% CI=0.71–

4.58) for the Rh incompatible offspring compared with the Rh compatible offspring; with the males exhibiting higher rate ratio

(RRadj=2.37; 95% CI=0.82–6.86) than the females (RRadj=0.93 95% CI=0.12–7.01). Among all offspring, the RRadj for ABO

incompatibility was lower and the elevated rate ratio was similarly limited to the males (RRadj=1.68; 95% CI=0.76–3.70). For

Rh and/or ABO incompatibility, the RRadj was 1.57 (95% CI=0.87–2.82). A statistically significant result was detected only for

the male offspring (RRadj=2.22; 95% CI=1.10–4.47).

Conclusion: Although the results should be interpreted with caution given the few events of SSD, the findings extend the line

of evidence that maternal–fetal blood incompatibility is a risk factor for schizophrenia spectrum disorder; with the strongest

evidence to date implicating that the susceptibility pertains only to male offspring.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Schizophrenia; Schizophrenia spectrum disorder; Maternal–fetal blood incompatibility; Rh incompatibility; ABO incompatibility

0920-9964/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.schres.2005.06.005

* Corresponding author.

E-mail address: [email protected] (B.J. Insel).

1. Introduction

Mounting empirical evidence for a neurodevelop-

mental origin of schizophrenia is fostering research on

80 (2005) 331–342

B.J. Insel et al. / Schizophrenia Research 80 (2005) 331–342332

the possible prenatal contribution to the risk of schizo-

phrenia (Weinberger, 1987; Susser et al., 1999; Mur-

ray and Fearon, 1999; McGrath et al., 2003). While

epidemiologic findings for individual prenatal risk

factors are not definitive, the evidence collectively

suggests that prenatal insults are of etiological signif-

icance in schizophrenia.

In the present study, we hypothesized that offspring

of a maternal–fetal blood incompatible pregnancy are

at increased risk for schizophrenia, based on biolog-

ical plausibility and prior reports (see below). In

maternal–fetal blood incompatibility, an elevated

risk of schizophrenia in the offspring is postulated to

arise from a maternal immune response to the pater-

nally inherited foreign fetal red blood cell antigens.

Disruption of fetal neurodevelopment may evolve

from multiple immunologic pathways leading to sev-

eral pathological states including hemolytic induced

chronic fetal hypoxia (Mollison et al., 1993), the

accumulation of neurotoxic bilirubin within the devel-

oping brain (Cashore, 1990), as well as more specu-

lative pathological processes involving the cross-

reaction of maternal IgG antibodies with fetal brain

antigens (Laing et al., 1995), and the breakdown of

maternal immune tolerance of the fetus (van Gent et

al., 1997).

Rh and ABO maternal–fetal blood incompatibil-

ity both can induce the production of maternal IgG

antibodies against the fetal red blood cell antigens.

The IgG antibodies can in turn transverse the pla-

centa, enter the fetal circulation and attack fetal red

blood cells (Bowman, 1999), and also gain access

to the developing brain due to the immaturity and

the permeability of the blood brain barrier (Adinolfi,

1985). A significant maternal antibody response can

provoke hemolytic disease of the fetus. An Rh-

negative woman requires a sensitizing exposure to

Rh-positive cells. As a result, firstborn Rh incom-

patible offspring rarely experience hemolytic dis-

ease, while second and later Rh incompatible

offspring are more likely to suffer its immunological

sequalae. In contrast, since most group O women

have anti-A and anti-B antibodies predating preg-

nancy, ABO hemolytic disease can occur in first

pregnancies (Rawson and Abelson, 1960; Ozolek et

al., 1994). ABO and Rh incompatibility are the two

most common causes of hemolytic disease of the

fetus. Rh disease is associated with serious morbid-

ity and mortality while ABO disease is consistently

milder.

Rh incompatibility, but not ABO incompatibility,

has been examined as a risk factor for schizophrenia

in several studies. A historical cohort study, con-

ducted by Hollister et al. (1996) within a Danish

sample born between 1959 and 1961, was the first

study to explore the hypothesis. The authors restrict-

ed the analyses to males due to the small number of

females diagnosed with schizophrenia within the

cohort. They found that among the men, the rate

of schizophrenia was significantly greater in the Rh

incompatible group than in the Rh compatible group

(RR=2.78, 95% confidence interval=1.2– 6.6). The

risk was confined to second and later-born offspring

from the Rh incompatible pregnancies. The study

had some limitations: the restriction to male subjects,

the use of hospital registry diagnoses for schizophre-

nia, and subject eligibility that was contingent on

neonatal blood typing, which was not a routine

procedure at that time. To further explore this asso-

ciation, Palmer et al. (2002) conducted a family-

based candidate-gene study using patient–parent(s)

pairs and trios comprising the youngest affected

male or female sibling and at least one biological

parent. The families were drawn from a Finnish

sample born prior to the widespread use of anti-Rh

(D) prophylaxis. Their estimated relative risk for Rh

maternal–fetal genotype incompatibility was 2.6

(90% CI=1.1, 6.0). Kraft et al. (2004) extended

the work of Palmer et al. (2002) using the same

Finnish sample and included all affected siblings

within a family. They reported a relative risk of

1.7 (90% CI=1.1, 2.5) in second and later born Rh

incompatible offspring, based on the assumption that

firstborn incompatibles were not at increased risk.

While the sample of Palmer et al. (2002) and Kraft

et al. (2004) appeared to contain a sufficient sample

of females (respectively 38% and 42% of the entire

sample), the effect estimates were not differentially

reported by sex of affected offspring.

The Rh incompatibility–schizophrenia association

has also been tangentially included in epidemiologic

studies that investigated the etiologic contribution of

obstetric complications. Two meta-analyses (Geddes

et al., 1999 and Cannon et al., 2002) examined the

association within mixed-sex samples, in which both

excluded the study of Hollister et al. (1996). Assem-

B.J. Insel et al. / Schizophrenia Research 80 (2005) 331–342 333

bling ten studies, Geddes et al. (1999) calculated a

summary odds ratio of 1.40 (95% CI=0.54– 3.63) for

Rh incompatibility and Cannon et al. (2002), aggre-

gating three other studies, derived a summary odds

ratio of 2.00 (95% CI=1.01– 3.96) for the effect of

their Rhesus variable. (The studies evaluated different

Rhesus related factor. Sacker et al. (1995), Kendell et

al. (2000), and Byrne et al. (2000) estimated respec-

tively the effect of maternal Rh negativity, Rh anti-

bodies, and Rh incompatibility).

The epidemiologic literature on schizophrenia has

consistently shown that males and females differ in

terms of premorbid functioning, age at onset, symp-

tomatology, treatment response, and course (Gold-

stein, 1997). Similarly, male infants are more

severely affected by Rh hemolytic disease and kernic-

terus than their female counterparts (Walker and Mol-

lison, 1957; Ulm et al., 1998). Based on these

observations and from the study of Hollister et al.

(1996), we posited that sex of the offspring would

modify the effect of maternal–fetal blood incompati-

bility on the risk of schizophrenia.

We examined the effect of Rh and ABO incompat-

ibility in offspring from the Prenatal Determinants of

Schizophrenia (PDS) study (Susser et al., 2000), a

prospective study drawn from a large birth cohort

born before the availability of anti-Rh (D) prophylax-

is. The PDS study featured comprehensive measures

of prenatal and perinatal exposures, including the

blood types of mother and infant, and implemented

rigorous diagnostic assessments of adult schizophre-

nia outcome in the offspring. In addition, the cohort

had an appreciable number of female offspring, which

enabled us to examine interaction between Rh and

ABO incompatibility and sex of offspring on the risk

of schizophrenia.

2. Methods

2.1. Description of the cohort

The analytic sample was derived from the Prenatal

Determinants of Schizophrenia (PDS); which has

been described in detail elsewhere (Susser et al.,

2000). The PDS study originated from the Child

Health and Development Study, (CHDS), a birth co-

hort implemented to investigate associations between

pregnancy events and offspring development (van den

Berg, 1979; van den Berg et al., 1988). From 1959 to

1967, the CHDS assembled 19044 live births of

women who received prenatal care from Kaiser Per-

manente Plan-Northern California Region (KP). The

cohort was racially, educationally and occupationally

diverse and reflected the population of KP and of the

population of Alameda County during the course of

the study.

The PDS study cohort consisted of the 12094

liveborn offspring who belonged to KP anytime

from January 1, 1981 (the year in which computerized

registries became available) to December 31, 1997.

The cohort was followed for seventeen years and

consequently, the ages of the offspring ranged from

13 to 38 over the course of the PDS study follow-up.

All subjects in the PDS study provided written

informed consent for human investigation. The

study protocol was approved by the Institutional Re-

view Boards of the New York State Psychiatric Insti-

tute and the KP.

2.2. Follow-up

The KP registries provided a record of each off-

spring’s membership status and treatment status over

the 17 year follow-up. Offspring who remained in KP

and those lost to follow-up were similar to one another

on sociodemographic factors, prenatal exposures, and

birth outcomes (Susser et al., 2000). Computerized

psychiatric and medical utilization records were used

to track offspring who were treated for psychiatric

disorders while they were members of the KP.

2.3. Diagnoses

Schizophrenia spectrum disorders were defined as

schizophrenia, schizoaffective disorder, delusional

disorder, psychotic disorder not otherwise specified,

and schizotypal personality disorder (Kendler et al.,

1995). Potential cases among the cohort were identi-

fied by means of computerized record linkage be-

tween CHDS and KP inpatient, outpatient, and

pharmacy registries. Offspring with schizophrenia or

potential schizophrenia spectrum disorders were

screened from ICD-9 registry diagnoses of 295–299

or medical records indicating antipsychotic medica-

tions, followed by psychiatrist review of all records.


Among those who screened positive (N =183), 13

were deceased.

Contact was achieved with 146 (86%) of the sur-

viving 170 potential cases and 107 completed in-

person diagnostic assessment. Clinically experienced

interviewers were trained to administer the Diagnostic

Interview for Genetic Studies (DIGS) (Nurnberger et

al., 1994). Diagnoses were assigned using DSM-IV

criteria by consensus of three psychiatrists based on

review of the written DIGS narrative, the medical

records, and discussions with the interviewers. Over-

all, 71 PDS offspring met the criteria for SSD, 44 as

result of consensus diagnosis following interview and

27 from chart reviews (90% of cases had either

schizophrenia or schizoaffective disorder).

2.4. Assessment and definition of blood

incompatibility

As stated above, a comprehensive interview was

conducted at a woman’s enrollment into the CHDS. In

addition to the maternal interview and medical chart

extraction, blood samples were drawn. The primary

source for the maternal–fetal blood incompatibility

exposure was the assay results derived from the ma-

ternal and newborn blood specimens collected at de-

livery and analyzed at the same laboratory.

Maternal–fetal Rh incompatibility is conventional-

ly defined as a pregnancy in which an Rh-negative

woman has an Rh-positive fetus. However, predicated

on the fact that a first Rh incompatible pregnancy

immunizes the mother but rarely suffers from its

immunologic sequelae and findings of Hollister et

al. (1996), we restricted our definition of Rh exposure

to second and later Rh incompatible pregnancy (re-

ferred to as strict Rh). Due to the lack of data on prior

Rh incompatible pregnancies, the number of past Rh

incompatible pregnancies was approximated by the

number of all past pregnancies. This surrogate mea-

sure is reasonable because among Rh-negative women

whose indexed pregnancy was Rh incompatible, the

probability that a past pregnancy was also Rh incom-

patible is over 70%, assuming that the father of the

indexed pregnancy also fathered at least one previous

pregnancy. (Under the assumption that the frequency

of Rh-positive blood in a sample is 85%, 45% of Rh-

positive persons are homozygous for D and 55% are

heterozygous. If a father is homozygous, all of his

children are Rh-positive and if he is heterozygous,

each child has 50% chance of being Rh-positive. As a

result, 0.45+0.5�0.55=0.725 of offspring are Rh-

positive. A higher frequency of Rh positive yields a

higher proportion of Rh-positive offspring).

ABO incompatibility was defined as a pregnancy

in which a pregnant woman has type O blood and the

fetus has blood type A or B. We did not adjust for the

existence of a prior ABO incompatible pregnancy.

We also combined the two distinct exposures,

strict Rh and ABO incompatibility, into a single

exposure variable (referred to as composite blood

incompatibility). Our rationale was that both types

of maternal–fetal blood incompatibility give rise to

the production of transplacentally transmitted mater-

nal antibodies, which can cause hemolytic disease of

the fetus, our primary hypothesized causal mecha-

nism for schizophrenia.

2.5. Analytic strategy

The construction of the PDS analytic sample is

described in detail elsewhere by Susser et al. (2000).

Since the CHDS birth cohort contained siblings, only

one sibling from each family was randomly selected

to maintain independence of observations in the anal-

yses. Due to the limited number of offspring diag-

nosed with SSD during the course of the PDS study

follow-up, if a sibship contained an affected sibling,

the affected sibling was retained and the unaffected

siblings were excluded. However, if a sibship did not

include an affected sibling, then one unaffected sib-

ling was randomly selected for inclusion into the

sample. This selection process resulted in 7796 off-

spring. In the present analyses, five offspring diag-

nosed with SSD were subsequently excluded; four

who were diagnosed prior to January 1, 1981 (the

start date of the PDS study) and one who had an

affected sibling who was a member of the PDS cohort.

Analyses that included these five subjects resulted in

equivalent findings (Insel, 2003).

Rh and ABO compatibility status were unavailable

respectively for 758 (9.7%) and 998 (12.8%) of the

7791 PDS offspring. Missing blood types were highly

correlated (kappa=0.75), which reduced the possibil-

ity of differential ascertainment of Rh incompatible

compared with ABO incompatible pregnancies. Prior

to the main analysis, we evaluated the impact of


missing blood type data by examining its correlation

with maternal health conditions and birth outcomes

such as low birth weight and prematurity; no correla-

tions were observed. Thus, it is unlikely that substan-

tial bias would have been introduced by the exclusion

of offspring with missing blood compatibility status.

Different subsamples were constructed for each of

the blood compatibility analyses. The Rh analyses

focused on only second and later born unrelated off-

spring. From among the 7791 PDS offspring, 7033

offspring had identifiable Rh compatibility status. The

exclusion of the 1832 firstborns produced an Rh

analytic sample of 5201 offspring in which 48 were

diagnosed with SSD. The ABO analytic sample

contained 6793 offspring in which 59 offspring were

diagnosed with SSD. To construct the sample for the

composite blood incompatibility analysis, firstborns

were retained since ABO incompatible firstborns

were regarded as exposed due to lack of the necessity

of a prior sensitizing pregnancy to produce hemolytic

disease. We regarded the Rh incompatible firstborns

to be unexposed. However, to control for the possi-

bility that the Rh incompatible firstborns were to some

extent exposed; we inserted an additional exposure

category reflecting Rh incompatible firstborns in the

composite blood analytic models. Among the 7791

offspring, 681 were missing the composite blood

compatibility status, 77 ABO status, and 317 Rh

status; yielding a composite blood sample of 6716

offspring in which 59 offspring were diagnosed with

SSD (refer to Table 1). Nevertheless, we also ran the

analyses on a composite blood incompatible sample

excluding all firstborns regardless of compatibility

status and obtained essentially the same results.

Cox Proportional Hazards regression (Cox and

Oakes, 1984) was used to analyze the data since this

statistical technique takes into account varying dura-

tions of follow-up, while simultaneously adjusting for

Table 1

Distribution of schizophrenia spectrum disorder cases

Analytic sample Total sample

size

Unexposed

offspring

Exposed

offspring

Noncases Cases Noncases Cases

Strict Rh 5201 4808 43 345 5

ABO 6793 5818 49 916 10

Composite blood

incompatibility

6716 5448 44 1209 15

multiple covariates. For offspring diagnosed with

SSD, the onset date of SSD was approximated by

the date of first hospital admission or first outpatient

visit. Thus, the length of the follow-up for offspring

with SSD was quantified in days elapsed from the

beginning of observation until the onset date of SSD.

Analogously, for unaffected offspring, it was calculat-

ed as days since the start of the observation until the

KP membership termination date or until the end of

the PDS study, whichever date came first.

To strengthen the evidence for causality, confound-

ing needs to be excluded via examination of the

association within the context of potentially related

covariates (i.e., those correlated with blood incompat-

ibility and independently related to the diagnosis of

SSD). Based on the uniqueness of the blood incom-

patibility exposure, maternal ethnicity and maternal

age were selected as potential confounders. We also

assessed whether other variables such as maternal

education could modify the results and none had an

appreciable impact. This analytical process was re-

peated for the ABO and the composite blood incom-

patibility samples.

The current tenet of epidemiologic textbooks is

that an additive model is the most appropriate method

to identify biological interaction. Accordingly, we

adapted an approach derived from Darroch (1997)

and Rothman and Greenland (1998) to investigate

the biological interaction between maternal–fetal

blood incompatibility and sex of the offspring using

an additive model. Interaction is supported if the

combined effect of blood incompatibility and sex of

the offspring on the rate of schizophrenia is greater

than the sum of their independent effects. To statisti-

cally assess whether the interaction was significant,

we employed a test devised by Ten Have et al. (2002)

that identified an interaction effect as being significant

if the expected combined rate ratio is less than the

lower limit of the confidence interval of the observed

combined effect.

3. Results

3.1. Rh Sample characteristics

The Rh compatible group contained 5201 offspring and

the Rh incompatible group had 350 offspring. The Rh

Table 2

Sociodemographic characteristics of study sample for the Rh analysis

Preliminary Rh

sample (n =7033)

Final Rh samplea

(n =5201)

Unexposed Rh

samplea (n =4851)

Exposed Rh

samplea (n =350)

n % n % n % n %

Maternal characteristics

Age at delivery

b20 years 385 5.5 135 2.6 128 2.6 7 2.0

20–29 4113 58.5 2765 53.2 2578 53.1 187 53.4

30–39 2202 31.3 1988 38.2 1856 38.3 132 37.7

40+ 333 4.7 313 6.0 289 6.0 24 6.9

Mean age (SD) 27.9 (6.2) 29.2 (6.0) 29.2 (6.0) 29.2 (5.9)

Race/ethnicity

White 3935 56.0 2783 53.5 2547 52.5 236 67.4

Black 2028 28.8 1621 31.2 1536 31.7 85 24.3

Other 1070 15.2 797 15.3 768 15.8 29 8.3

Number of prior pregnancies

None 1832 26.0 0 0.0 0 0.0 0 0.0

One 1690 24.0 1690 32.5 1583 32.6 107 30.6

Two or more 3510 50.0 3511 67.5 3268 67.4 243 69.4

Infant characteristics

Sex

Male 3602 51.2 2643 50.8 2462 50.8 181 51.7

Female 3431 48.8 2558 49.2 2389 49.2 169 48.3

Low birth weight (b=2500 g)

Absent 6657 94.7 4923 94.7 4594 94.7 329 94.0

Present 376 5.3 278 5.3 257 5.3 21 6.0

Mean birth weight in oz (SD) 117.1 (18.2) 117.9 (18.4) 117.9 (18.4) 117.9 (18.6)

Prematurity (b37 weeks)

Absent 6522 92.7 4809 92.5 4484 92.4 325 92.9

Present 511 7.3 392 7.5 367 7.6 25 7.1

Mean gestational age (SD) 278.9 (14.2) 278.4 (14.3) 278.4 (14.2) 278.8 (15.0)

ABO status

ABO compatible 5790 82.3 5492 82.2 3990 82.3 298 85.1

ABO incompatible 926 13.2 874 13.1 654 13.5 52 14.9

Missing 317 4.5 317 4.7 207 4.3 0 0

a Excludes all firstborns.


incompatible offspring in the PDS study were similar to the

Rh compatible offspring, except that Rh incompatible off-

spring were more likely to be offspring of Caucasian women

than offspring of African-American and other minority eth-

nic women (see Table 2).

Table 3

Rate ratios of schizophrenia spectrum disorder and strict Rh incompatibil

SSD count (exposed) All offspring (n =5201)

48 (5)

RR 95% CI

Unadjusted 1.63 0.65–4.12

Adjusteda 1.80 0.71–4.58

a Adjusted for maternal age and maternal ethnicity.

3.2. The effects of Rh incompatibility

The unadjusted rate ratio for strict Rh incompatibility

and SSD was 1.63 (95% CI=0.65–4.12) (Table 3). Adjust-

ing for maternal age and ethnicity modestly increased the

ity

Male offspring (n =3602) Female offspring (n =2558)

30 (4) 18 (1)

RR 95% CI RR 95% CI

2.02 0.71–5.79 0.90 0.12–6.74

2.37 0.82–6.86 0.93 0.12–7.01

Table 4

Rate ratios of schizophrenia spectrum disorder and strict Rh incom-

patibility: effect modification for sex of offspring

Study sample (n =5201)

Combined effect of Adjusted RRa 95% CI

Being male Strict Rh incompatible

No No 1.00

Yes No 1.51 0.82–2.78

No Yes 0.94 0.13–7.11

Yes Yes 3.50 1.17–10.49



rate ratio to 1.80 (95% CI=0.71–4.58). Stratification on sex

produced an adjusted rate ratio of 2.37 for strict Rh incom-

patibility among the male offspring, but did not achieve

statistical significance (95% CI=0.82–6.86). In contrast,

no elevated effect was observed for the female offspring.

Although not shown in the tables, for comparative purpose

with the existing literature, we conducted analyses using an

alternative definition for the Rh incompatibility exposure,

which considered all Rh incompatibles as exposed regard-

less of birth order. This resulted in slightly lower adjusted

RR=1.67 (95% CI=0.71–3.88) than for the strictly defined

Rh incompatibility exposure. Analogous to our prior finding

for the male offspring, we derived an adjusted rate ratio of

2.33 (95% CI=0.90–6.02).

3.3. Interaction between Rh incompatibility and sex of

offspring

To examine interaction between sex of offspring and Rh

incompatibility, we computed the combined effects of being

male and being strictly Rh incompatible on the incidence of

SSD, as shown in Table 4. The adjusted rate ratio for the

independent effect of male sex alone (i.e., the effect among

Rh compatibles) was 1.51 (95% CI=0.82–2.78) and for the

independent effect of strict Rh incompatibility (i.e., the

effect among women) was 0.94 (95% CI=0.13–7.11). The

adjusted rate ratio for the observed combined effect of male

sex and strict Rh incompatibility was 3.50 (95% CI=1.17–

10.49). We calculated the expected effect for the scenario of

Table 5

Rate ratios of schizophrenia spectrum disorder and ABO incompatibility

SSD count (exposed) All offspring (n =6793)

59 (10)

RR 95% CI

Unadjusted 1.31 0.66–2.59

Adjusteda 1.28 0.65–2.53


no interaction within an additive model, which is the sum of

the two independent effects minus the reference value of 1

(1.51+0.94�1.00=1.45). The observed combined effect of

3.50 was 2.4 times the expected combined effect of 1.45,

suggesting the presence of additive interaction. Nonetheless,

the data did not satisfy the criterion of Ten Have et al.

(2002) for a significant interaction effect since the expected

combined effect for the absence of interaction was greater

than 1.17, the lower limit of the 95% confidence interval of

the observed combined effect.

3.4. The effects of ABO incompatibility

The sociodemographic characteristics for the ABO sam-

ple were almost identical to those of the Rh sample. The rate

ratios for ABO incompatibility were consistently lower than

the corresponding rate ratios for Rh incompatibility (see

Table 5). Among the male offspring, the adjusted rate

ratio for ABO incompatibility was 1.68 (95% CI=0.76–

3.70). Analogous to the female Rh incompatibility results,

no elevation of risk for ABO incompatibility for the female

offspring was ascertained.

We also examined interaction between sex of offspring

and ABO incompatibility. The adjusted rate ratio for the

independent effect of male sex alone (i.e., the effect among

ABO compatibles) was 1.33 and for the independent effect

of ABO incompatibility (i.e., the effect among women) was

0.62. The adjusted rate ratio for the observed combined

effect of male sex and ABO incompatibility was 2.29

(95% CI= 1.01–5.17) and the expected combined effect in

absence of interaction was 0.95. These results satisfied the

criterion of Ten Have et al. (2002) for statistical significant

interaction effect since the expected combined effect in

absence of interaction was less than the lower limit of the

95% confidence interval for the observed combined effect.

3.5. The effect of composite blood incompatibility

We next examined whether having the composite (either

Rh or ABO) blood incompatibility increased the risk for

SSD. The adjusted rate ratio for composite blood incompat-

ibility was 1.57 (95% CI=0.87–2.82), as shown in Table 6.

Male offspring (n =3467) Female offspring (n =3326)

36 (8) 23 (2)

RR 95% CI RR 95% CI

1.83 0.83–4.02 0.61 0.14–2.61

1.68 0.76–3.70 0.62 0.14–2.63

Table 6

Rate ratios of schizophrenia spectrum disorder and composite blood (strict Rh and/or ABO) incompatibility

SSD count (exposed) All offspring (n =6716) Male offspring (n =3425) Female offspring (n =3291)

59 (15) 36 (12) 23 (3)

RR 95% CI RR 95% CI RR 95% CI

Unadjusted 1.55 0.86–2.79 2.24 1.12–4.48 0.70 0.21–2.34

Adj. for covariatesa 1.57 0.87–2.82 2.22 1.10–4.47 0.70 0.21–2.35

a Adjusted for maternal age, maternal ethnicity, and Rh incompatible firstborns.


Stratification on sex of offspring revealed a statistically

significant two-fold rate ratio for blood incompatibility

among the male offspring, (Adjusted RR=2.22, 95%

CI=1.10–4.47).

We also evaluated the presence of interaction between

sex of offspring and composite blood incompatibility. The

corresponding adjusted rate ratio for the independent effect

of male sex was 1.20, for the independent effect of blood

incompatibility was 0.71, and the adjusted rate ratio for their

observed combined effect was 2.67 (95% CI=1.30–5.48)

(see Table 7). The expected combined effect in the absence

of interaction was 0.91. The observed combined effect was

almost three times the corresponding expected combined

effect and met the criterion of Ten Have et al. (2002) for

statistical significance.

7

Material-Fetal Blood Incompatibility RateRatios (RRadj) for SSD

io

4. Discussion

The study has extended the line of evidence that Rh

incompatibility may be a risk factor for schizophrenia.

We found that strict Rh incompatibility was associated

with a two-fold elevated rate of schizophrenia spectrum

disorder and the effect was limited to male offspring.

We also explored the effect of ABO incompatibility and

observed a weaker effect, also limited to male off-

spring. When Rh and ABO incompatibility were ana-

Table 7

Rate ratios of schizophrenia spectrum disorder and composite blood

(strict Rh and/or ABO) incompatibility: effect modification for sex

of offspring

Study sample (n =6716)

Combined effect of Adjusted RRa 95% CI

Being male Incompatible

No No 1.00

Yes No 1.20 0.66–2.17

No Yes 0.71 0.21–2.40

Yes Yes 2.67 1.30–5.48


lyzed separately, statistical significance was not

achieved. However, the composite Rh and/or ABO

incompatibility exposure produced a significant, great-

er than two-fold rate of schizophrenia among the male

offspring. The findings are summarized in Fig. 1.

Schizophrenia is currently regarded as a complex

disorder that arises from heterogeneous pathways in-

volving interactions between many different genes and

environmental factors, each with varying causal effec-

tivity. Thus, it may not be surprising that a modest

effect size was found for maternal–fetal blood incom-

patibility. The magnitude does not preclude a potential

causal association; rather the results are compatible

with etiological heterogeneity within the disorder.

Moreover, the effect size may reflect the prevalence

within the study population of other necessary factors

that operate in conjunction with maternal–fetal blood

incompatibility to cause schizophrenia.

We considered whether the findings were internally

valid or were the result of ascertainment bias, uncon-

trolled confounding, or chance. The prospective de-

1.68 2.220.93 0.65 0.702.370

1

2

3

4

5

6

Rh ABO Rh/ABO

Blood Incompatibility

Ad

just

ed R

ate

Rat

RRadj

Fig. 1. Gender differences in the effects of blood incompatibility for

SSD. Rate ratios and 95% confidence intervals are adjusted for mater-

nal age and ethnicity. Dashed line represents the null value of 1. Only

the male adjusted 95% CI for Rh/ABO is above the null value line.


sign of the PDS study makes it unlikely that the

findings could be accounted for by ascertainment

bias. The offspring were identified prior to their

birth from a well-characterized population of pregnant

women. The incidence of schizophrenia spectrum

disorder among the PDS cohort was ascertained me-

thodically and independently of prenatal and birth

outcomes. Furthermore, maternal and fetal exposure

data were collected uniformly and consistently at each

woman’s enrollment in the study, throughout the preg-

nancy, and during labor and delivery.

The association is also unlikely to be explained by

uncontrolled or residual confounding. We adjusted for

potential confounders, of which only two covariates,

maternal ethnicity and maternal age at delivery, had a

slight impact on the rate ratios. In addition, maternal–

fetal blood incompatibility is an immutable, biological

phenomenon that is determined at fertilization. Thus,

maternal behavior or lifestyle before, during, or after a

pregnancy cannot influence maternal–fetal blood in-

compatibility. Moreover, as predicted, adjusting for

other maternal and infant characteristics that were

observed in past studies to be risk factors for schizo-

phrenia did not alter the effect estimates for maternal–

fetal blood incompatibility.

Lastly, we considered the contribution of chance.

We observed elevated but not statistically significant

rate ratios for Rh and ABO incompatibility among the

male offspring. However, the associations were sup-

ported by the fact that the hypotheses were precisely

and a priori formulated. In addition, biological plau-

sibility, the Rh incompatibility findings of Hollister et

al. (1996) and Palmer et al. (2002), and our statisti-

cally significant findings among the males for the

composite blood incompatibility exposure strengthen

the validity of the associations. Nevertheless, chance

remains a possible explanation.

We also contemplated whether the lack of an ele-

vated risk for maternal–fetal blood incompatibility for

the female offspring was real or reflected the inade-

quacy of the study power. During the PDS study

follow-up, substantially more males than females di-

agnosed with SSD were ascertained. Using the multi-

national WHO-Determinants of Outcome-Study,

Hambrecht et al. (1992) estimated that approximately

90% of men are less than age 40 at diagnosis for

schizophrenia, while only 70% of women are less

than age 40 at diagnosis. The lack of power within

the PDS study sample may reflect the fact that females

are often diagnosed with schizophrenia three to five

years later than their male counterparts. Although we

found a later average age at diagnosis for SSD among

the Rh incompatible group compared with the Rh

compatible group in the PDS cohort, in which the

age difference was 2.7 years, the independent sample

t-test was not statistically significant ( p-value=0.24).

Thus, it is unlikely that Rh incompatibility was asso-

ciated with a later onset of schizophrenia in women

and limited our ability to detect an elevated effect for

the females. We also conducted sensitivity analyses

and the resultant female rate ratios for Rh incompat-

ibility (ranged from 0.90 to 1.27) never approached

the two-fold effect observed for the male offspring

and never achieved marginal statistical significance

(results available on request from first author).

Biological and empirical observations corroborate

the observed sex differences in our findings, which

revealed that only males showed the effect of blood

incompatibility for schizophrenia. Moreover, sex dif-

ferences have been consistently observed in the inci-

dence and course of numerous types of psychopathol-

ogy (Rutter et al., 2003). These findings indicate that

biological mechanisms contributing to causal hetero-

geneity may underlie the sex differences. Divergent

results may reflect the consequences of sexual dimor-

phism in brain structure and function and the interplay

of estrogen. A slower rate of cerebral development in

males than in females (Seeman, 1989) may augment

the susceptibility of the male brain to environmental

and perinatal insults. Male infants are also more se-

verely affected by maternal Rh-D immunization, in-

cluding kernicterus, than female infants (Ulm et al.,

1998; Walker and Mollison, 1957). In vitro and ani-

mal observations provide additional, although specu-

lative, support for the sex differential effect of

maternal–fetal blood incompatibility on schizophre-

nia. In cultured neurons, estrogen attenuated the neu-

rotoxic effects of oxidative damage from prolonged

exposure to hemoglobin (Sawada et al., 1998; Regan

and Guo, 1997). Similarly, in vivo findings showed

that male gerbils were more susceptible to ischemic

cortical and hippocampal injury than female gerbils

(Hall et al., 1991). To summarize, the cumulative

evidence from basic science and clinical research

suggests that male fetuses and infants may be more

vulnerable to certain early adverse environmental


exposures of maternal–fetal blood incompatibility and

less able also to compensate for aberrant neurodeve-

lopment than their female counterparts.

This is the first study to explore the effect of ABO

incompatibility in relation to schizophrenia. Although

both Rh and ABO blood incompatibility induce trans-

placentally transmitted maternal antibodies, they dif-

fer with respect to both biological sequelae and

clinical significance. An ABO incompatible pregnan-

cy occurs more frequently than an Rh incompatible

pregnancy, but is rarely accompanied by hemolytic

disease of the fetus. When ABO hemolytic disease

does occur, it is relatively mild. In contrast, to Rh

hemolytic disease, ABO hemolytic disease can also

arise in firstborn infants (Mollison et al., 1993; Wal-

dron and de Alarcon, 1999). The ABO results are

preliminary, but suggest that further research on

ABO incompatibility as a possible risk factor for

schizophrenia also warrants investigation.

4.1. Limitations of the study

The study has some limitations that deserve con-

sideration. The first limitation is the modest number of

offspring diagnosed with SSD during the course of the

PDS follow-up.

A second limitation is that approximately 10% of

the mother–infant dyads were missing blood incom-

patibility data. However, the likelihood of a bias

introduced from the exclusion of offspring with miss-

ing blood types is slight. Moreover, most blood col-

lection occurred routinely during delivery and missing

blood types were not correlated with prenatal factors

or birth outcomes.

A third limitation is the absence of information on

whether a mother experienced a prior Rh incompati-

ble pregnancy. As a result, we utilized the number of

all prior pregnancies to estimate the number of prior

Rh incompatible pregnancies. Based on the distribu-

tion of the Rh-negative frequency in the sample,

approximately 23% of presumed second and later

Rh incompatible offspring may actually have been

Rh incompatible firstborns since a prior pregnancy

may actually have been Rh compatible. Thus, our

operationalized measure may have overestimated the

number of second and later Rh incompatible pregnan-

cies and would bias the results toward the null by

misclassification of some unexposed offspring (first-

born Rh incompatible) as exposed offspring (second

and later Rh incompatible).

The lack of study power did not allow us to further

examine whether firstborn Rh incompatible offspring

are at elevated risk for schizophrenia since the PDS

study cohort contained only one Rh incompatible

firstborn offspring diagnosed with SSD. Past findings

concerning the risk for later schizophrenia among

firstborn Rh incompatible offspring were inconclu-

sive, including the findings of Hollister et al. (1996),

which observed an elevated but statistically insignif-

icant risk. The data of Kraft et al. (2004) equally

supported two scenarios, one in which all Rh incom-

patible offspring are at increased risk and the second,

in which only subsequent Rh incompatible offspring

are at increased risk for schizophrenia. The biological

plausibility is less robust for firstborn Rh incompati-

ble offspring compared with second and later Rh

incompatible offspring. Generally, during a first Rh

incompatible pregnancy, an appreciable maternal im-

munologic response and ensuing hemolytic disease of

the fetus does not occur. Nevertheless, the association

between firstborn Rh incompatible offspring and

schizophrenia deserves investigation.

Another limitation is that we lacked information on

both post-natal bilirubin levels and the incidence of

hemolytic disease in the infants. At some later point,

we hope to obtain access to this data. Thus, our

present findings are applicable only to Rh and ABO

blood incompatibility. If hemolytic disease goes un-

treated, it may progress to neonatal kernicterus; irre-

versible brain damage accompanied by long-term

neurological deficits, and is likely to be associated

with a substantial risk of later schizophrenia, if the

infant survives. Presumably, hemolytic disease of the

fetus would confer a greater risk for schizophrenia

than blood incompatibility alone and could also pro-

duce an elevated effect for female offspring. Conse-

quently, the association between hemolytic disease of

the fetus and newborn and schizophrenia deserves

future consideration.

A final limitation concerns the inherent character-

istics of the PDS study population. The PDS offspring

were born between 1959 and 1967 to women who

were enrolled in a prepaid health care plan. Although

anti-Rh (D) prophylaxis was not available until 1968

to prevent sensitization in Rh-negative women, mul-

tiple methods of exchange transfusion and photother-


apy were in use to treat infants born with hemolytic

disease. As such, the Rh incompatible infants from the

PDS study cohort had access to these treatment mo-

dalities, which may have controlled the severity of the

neurologic sequelae associated with hemolytic dis-

ease. However, women living in low income countries

have limited access to perinatal health care, particu-

larly anti-Rh (D) prophylaxis, may experience more

unfavorable outcomes associated with Rh and ABO

incompatibility than our study population. Thus, our

results may have underestimated the association be-

tween maternal–fetal blood incompatibility and

schizophrenia.

5. Conclusion

Maternal–fetal blood incompatibility is a unique

prenatal exposure that incorporates genetics, the dis-

cordant maternal and fetal blood genotypes, and the

intra-uterine environment, ensuing from the maternal

immunologic responsiveness and fetal compensatory

mechanisms. We observed that the most pronounced

effect on later schizophrenia was exhibited among

male offspring who were products of second and

later Rh incompatible pregnancies, but the findings

raise the possibility that other blood group incompat-

ibilities may be etiologically important in schizophre-

nia. Our maternal–fetal blood incompatibility results

are congruent with the epidemiologic literature of

schizophrenia and consistent with past studies that

showed that immunological disturbances of the fetal

environment, particularly the induction of maternal

antibodies, might lead to aberrant fetal neurodevelop-

ment, manifesting in later psychosis in the offspring.

Likewise, the observed differential effect by sex of

offspring for maternal–fetal blood incompatibility ex-

posure on the risk of schizophrenia mirrors the sex

differences observed for hemolytic disease of the fetus

and newborn.

Acknowledgements

This manuscript was supported by the following

grants: NIMH 1R01MH 63264-01A1 (A.S.B.), NIMH

1K02MH65422-01 (A.S.B.), NICHD NO1-HD-1-3334

(B. Cohn), NICHD NO1-HD-6-3258 (B. Cohn).

We also wish to acknowledge the following indi-

viduals for their contributions to this work: Barbara

van den Berg, M.D., Barbara Cohn, Ph.D., and the

late Jacob Yerushalmy, M.D. We also wish to thank

the Lieber Center for Schizophrenia Research, the

National Institute for Child Health and Development,

and the Public Health Institute, Berkeley, CA.

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