Obstetrics and Gynecology Clinic, Specialistic Medical Center Czechów in Lublin

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Maternal underweight and pregnancy outcome:

prospective cohort study

WIESŁAW MACIEJ KANADYS

Abstract

Introduction. Our purpose was to assess the effect of prepregnancy maternal underweight on pregnancy outcome and neonatal birthweight.

Material and method. In prospective observational cohort study we analysed pregnant women who were provided with prenatal care in our

institution. The obstetric performance and pregnancy outcome of 359 women with underweight, defined as body mass index (BMI) below 19.8,

according to the Institute of Medicine criteria were compared with matched control subjects of normal weight (BMI 19.8-26.0). Odds ratios

(OR) and 95% confidence interval (CI) were determined by using Mantel-Haenszel technique. A probability value of p < 0.05 was considered

statistically significant. Results. The mean (± SD) birth weight of the infants of underweight women was 131 ± 17 g significantly less

(p = 0.0002) than of infants of control subjects. In the group of underweight women as compared with normal-weight women, there was

increased risk of preterm delivery (OR 1.46), small for gestational age (SGA), and low birth weight (< 2500 g) infants (respectively: OR 2.11,

p < 0.05, and OR 1.83, p < 0.05). In contrast, in this group of women there was rare incidence of gestational diabetes mellitus (GDM) (OR

0.72), pregnancy induced hypertension (PIH) (OR 0.80), large for gestational age (LGA) infants (OR 0.51, p < 0.05), and macrosomic infants

(> 4500 g) (OR 0.33). Conclusion. Low maternal weight was associated with increased prevalence of preterm delivery and low birthweight.

It was noticed that in this group of women there was low incidence of GDM, PIH, and neonates with large birthweight.

Key words: maternal underweight, pregnancy outcome, infant birth weight

Introduction

Uncomplicated course of pregnancy, which resulted in

normal growth and development of the fetus, is dependent on

many factors, individual one as well as factors associated with

one another. In addition to influence of environment and gene-

tic predisposition one of the factors is maternal nutritional

status before and during gestation which is connected with

adequate transfer of metabolic substrates, to support fully

energy requirements of growing fetus [1-3]. Disturbance of

this status can effect on pregnancy outcome [4, 5].

In recent years, in connection with epidemic prevalence

of overweight and obesity among society in developed coun-

tries, most researchers paid attention to examine the associa-

tion between excessive prepregnancy weight and obstetric

complications [6-8]. Controversly, maternal underweight can

also influence on adverse perinatal outcomes [9-11].

Importance of optimum nutrition during pregnancy remai-

ned underscore by some investigators, who suggest that pre-

natal nutrient supply affects not only on growth of fetal tissues

but also on incidence of various chronic disorders during adult-

hood [12, 13]. Pregnancy is key period when maternal under-

weight can indelibly „programme” fetal physiology and meta-

bolism and consequently it can lead to systemic pathologies in

later life including diabetes mellitus, cerebrovascular disease

and hypertension [14]. Although the rationale for this hypothe-

sis has been challenged [15].

This study was undertaken to investigate the relationship

between maternal underweight and pregnancy outcome and

neonatal birthweight.

Material and methods

Participants. This prospective observational cohort study

included 1381women with singleton pregnancies between 16

to 45 years of age who received prenatal care in our institu-

tion. They lived within the geographical boundaries of the Cze-

chów district in Lublin. Prepregnancy nutritional status was

defined using body mass index [BMI = weight (kg)/height

(m2)], according to the Institute of Medicine (IOM) criteria

[16]: underweight < 19.8, normal weight 19.8-26.0. Women

with body mass index above 26.0 (overweight and obese) were

excluded from further analysis.

Study variables. Selected data used in this study were re-

corded during routine prenatal care visits and during hospitali-

zation for labor and delivery.

Prepregnancy weight was determined on the basis of

maternal reporting. The weight recorded at the last prenatal

visit was designated as the maternal weight at delivery. The

mean length of time between last prenatal visit and delivery

was 7.1 ± 6.2 days. Total gestational weight gain was calcu-

lated by subtracting the pregravid weight from the weight last

recorded.

Pregnancy-induced hypertension (PIH) included preeclam-

psia and gestational hypertension was classified according to

the National High Blood Pressure Education Program Wor-

king Group on High Blood Pressure in Pregnancy as hyperten-

sion after 20 weeks gestation [17]. Preeclampsia was defined

as a sustained elevated blood pressure higher than 140/90 mm

Hg and proteinuria $2+ in a urine protein test. Gestational

hypertension was diagnosed if the blood pressure met the

above mentioned criteria without the presence of proteinuria.

The screening test for gestational diabetes mellitus

(GDM) was based on two-step approach. Women underwent

1h 50 g glucose challenge test (OCT) between 24 and 28 we-

eks of pregnancy. If the plasma glucose value at 1 h was $

7.8 mmol/l ($140 mg/dl), a patient was asked to undergo a 2-h

W. M. Kanadys24

75 g oral-glucose-tolerance test (OGTT) after an overnight

fast. GDM was diagnosed when glucose value at 2 h exceeded

7.8 mmol/l (>140 mg/dl) [18].

Neonatal outcomes included preterm delivery (before 37

weeks’ gestation), low birth weight (less than 2500 g), small

for gestational age (SGA) birth weight below the 10th percen-

tile for gestational age (2800 g for examine population), large

for gestational age (LGA) birth weight above the 90th percen-

tile for gestational age (4000 g for examine population), and

macrosomia (more than 4500 g).

Statistical analysis. Data were analyzed in 2 groups accor-

ding to prepregnancy BMI categories: underweight (<19.8),

and normal weight (19.8-26.0). Descriptive statistics included

means and standard deviations (SDs) for continuous variables

and frequency distributions for categorical variables. Student’s

independent t test and the chi-square test were used to test

differences between continuous and categorical variables,

respectively. Mantel-Haenszel technique was used to estimate

odds ratios (ORs) and 95% confidence intervals (95% CIs) [19].

P < 0.05 was considered statistically significant.

Table 1. Demographic characteristics among underweight and normal-weight women

BMI < 19.8

(n = 359)

BMI 19.8-26.0*

(n = 845)P value

Maternal age (years) 23.4 ± 4.0 25.5 ± 5.2 <0.0001

#19 55 (15.3) 84 (9.9) 0.0101

20-29 275 (76.6) 587 (69.5) 0.0146

$30 29 (8.1) 174 (20.6) <0.0001

Height (cm) 164.3 ± 5.8 163.4 ± 5.7 0.009

Pregravid weight (kg) 50.0 ± 4.5 59.6 ± 6.2 <0.0001

Pregravid BMI (kg/m2) 18.5 ± 1.0 22.3 ± 1.7 <0.0001

Gestational weight gain (kg) 14.8 ± 7.7 13.9 ± 6.9 0.0672

Parity 0.5 ± 0.8 0.7 ± 1.0 <0.0001

Nulliparity 246 (68.5) 448 (53.0) <0.0001

Data presented as mean ± SD or n (%);

* – the reference group; BMI – body mass index

Table 2. Comparisons of selected pregnancy complications and outcomes

for women with underweight and normal weight

BMI <18.8

(n = 359)

BMI 19.8-26.0*

(n = 845)OR (95% CI)

Gestational diabetes mellitus 9 (2.5) 29 (3.5) 0.72 (0.34-1.54)

Pregnancy-induced hypertension 12 (3.3) 35 (4.1) 0.80 (0.41-1.56)

Preterm delivery 26 (7.2) 43 (5.1) 1.46 (0.88-2.41)

Infant birth weight (g) 3265 ± 486 3396 ± 503 p = 0.0002

Birth weight category

Low birth weight (< 2500) 19 (5.3) 25 (3.0) 1.83 (1.00-3.37)

Small for gestational age 56 (15.6) 68 (8.0) 2.11 (1.45-3.08)

Large for gestational age 17 (4.7) 75 (8.9) 0.51 (0.30-0.88)

Macrosomia (> 4500) 2 (0.6) 14 (1.7) 0.33 (0.08-1.47)

Data – means ± SD or n (%);

* – reference group; OR – odds ratio; CI – confidence interval

Results

One thousand three hundred and eighty-one pregnant wo-

men participated in the study. Of these, 359 women (25.9%)

were underweight, and 845 subjects (61.2%) were normal

weight. The maternal demographics were significantly diffe-

rent among the women gathered by pre-pregnancy BMI. Com-

pared to women with a normal weight, the underweight wo-

men were significantly younger, had a significantly decreased

mean BMI, and they had lower parity, than the normal weight

women (Table 1).

The mean (± SD) birth weight of the infants of under-

weight women was 131 ± 17 g significantly less (p = 0.0002)

Maternal underweight and pregnancy outcome: prospective cohort study 25

than of infants of control subjects. In the group of underweight

women as compared with normal-weight women, there was in-

creased risk of preterm delivery (OR, 1.46; 95% CI, 0.88-2.41),

small for gestational age (SGA), and low birth weight

(< 2500 g) infants (respectively, OR, 2.11; 95% CI, 1.45-3.08,

and OR, 1.83; 95% CI, 1.00-3.37). In contrast, in this group of

women there was rare incidence of gestational diabetes mel-

litus (GDM) (OR, 0.72; 95% CI, 0.34-1.54), pregnancy induced

hypertension (PIH) (OR, 0.80; 95% CI, 0.41-1.56), large for

gestational age (LGA) infants (OR, 0.51; 95% CI, 0.30-0.88),

and macrosomic infants (>4500 g) (OR, 0.33; 95% CI, 0.08-

1.47) – Table 2).

Discussion

This study has demostrated that some adverse outcomes

of pregnancy are associated with low prepregnancy weight. We

found that underweight women showed increased risk for SGA,

and low birth weight infants in comparison to normal weight

patients. Our analysis confirms previous reports [20-26].

The biologic mechanisms underlying the association bet-

ween maternal undernutrition status and slower fetal growth

and development remain speculative [27, 28]. Ross et al. found

that women with underweight had a smaller plasma volume,

lower cardiac output, increases in peripherial vascular resistan-

ce, and lower rennin-aldosterone response in pregnancy com-

pared with normal-weight women. It seems probable that such

inadequate maternal hemodynamic adjustments may be asso-

ciated with uteroplacental insufficiency and the increased pre-

valence of small for gestational age babies observed [29, 30].

On the other hand recent research indicates that activity

of placental transport mechanisms may be directly modulated

by maternal nutrition [31]. Moderate malnutrition during

pregnancy caused a increase in maternal-fetal capacity for

glucose transport, which was at least partly explained by an

increase in total glucose transporter (GLUT) abundance.

These responses allow to sustain normal fetal growth, despite

chronic maternal hypoglycaemia and a decrease in the ma-

ternal-fetal gradient in arterial plasma glucose concentration

[32]. During chronic nutrient deficiency the development of

profound fetal hypoglycaemia helps to sustain the maternal-

fetal gradient in glucose concentration by restricting the re-

verse transfer of glucose to the placenta, and reducing pla-

cental glucose consumption [33]. There is induced fetal gluco-

neogenesis, what consequence is reduced fetal tissue protein

synthesis and slowing of fetal growth to a rate that can be

sustained by the reduced placental nutrient supply [31]. It is

necessary to stress that majority of underweight women were

not suffering severe dietary restriction.

Our analysis also showed increases in the frequency of

spontaneous preterm delivery in underweight group, but this

was not statistically significant. This finding is in accordance

with results of others [20-22, 25, 34-37].

The causes of most cases of preterm delivery remain un-

determined, and also the nature of the association between

prepregnancy malnutrition and preterm delivery is unclear.

Although low body mass index, which reflects an imbalance

between energy intake and energy expenditure, might be a ge-

neral marker of minimal tissue reserves. Frentzen et al. found

increased ketonuria, possible indicator of nutritional deficien-

cy, among women of delivering preterm, compared with con-

trol subjects [38]. Unlike previous reports, Ronnenberg et al.

found no association between maternal BMI and preterm birth

in this group of women [39].

Simultaneously in carried out study we observed decre-

ased incidence of LGA and macrosomic neonates. Other au-

thors made similar observations [22, 23, 25, 40].

Compared with normal weight pregnants, in women with

prepregnant underweight we also showed decrease in frequen-

cy of gestational diabetes mellitus and pregnancy-induced

hypertension, which is reflected in literature [22, 25, 40].

Although Villena-Heinsen et al. observed significant increase

in incidence of PIH [21].

In conclusion, low maternal weight was associated with

increased prevalence of preterm delivery and low birthweight.

It was noticed that in this group of women there was low

incidence of GDM, PIH, and neonates with large birthweight.

Presented at the 29th Congress of the Polish Gynaecological Asso-

ciation, held in Poznań, Poland, September 28-30, 2006.

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J W. M. Kandys

Obstetrics and Gynecology Clinic

Specialistic Medical Center Czechów in Lublin

ul. Kompozytorów Polskich 8

20-848 Lublin, Poland