the amniotic fluid index in late pregnancy

The amniotic fluid index in late pregnancy

R. H. Stigter, E. J. H. Mulder, H. W. Bruinse and G. H. A. Visser

Department of Perinatology and Gynecology, University Medical Center Utrecht, The Netherlands

Objective: Our purpose was to examine changes in the amniotic fluid index (AFI) in accuratelydated term pregnancies both in relation to gestational age and in relation to the onset ofspontaneous labor.Study design: This was a prospective observational study in 137 women with uneventful termpregnancies, in whom 220 AFI measurements were performed. More than one AFI value was avail-able from 51 individuals.Results: The AFI did not change significantly between 37 and 42 weeks’ gestation, but a significantreduction was seen during the last 11 days before the spontaneous onset of labor (R = -0.37, n = 83,p < 0.001). The AFI (corrected for gestational age) within individuals remained stable over periodsof up to 2 weeks. Meconium staining of the amniotic fluid was related to gestational age, but not tothe AFI or fetal distress at birth. No significant correlation was found between fetal distress and theAFI, or between fetal distress and the reduction in AFI during the last two measurements beforelabor.Conclusions: The reduction of the AFI in pregnancies progressing beyond term is related to thelabor process itself rather than to the exact gestational age.

Key words: AMNIOTIC FLUID INDEX, TERM PREGNANCY, ULTRASOUND, LABOR

INTRODUCTION

Attempts to determine the quantity of amniotic fluid inhuman pregnancy have been made by measuring theamount drained at hysterotomy during the first half, andby various dilution methods during the second half ofpregnancy. Normal values of amniotic fluid volume in thecourse of pregnancy have been produced by combiningdata from a number of published studies, and these show arise of the amniotic fluid volume to a maximum at 33–34weeks, followed by a gradual reduction1.

More recently, ultrasonographic techniques have beenused to determine the quantity of amniotic fluid. Geirssoncalculated the entire intrauterine volume by serial parallelplanimetric area measurements2. Phelan and colleaguesintroduced the amniotic fluid index (AFI), the sum of thedeepest amniotic fluid pockets measured in four abdominalquadrants, as a more practical semi-quantitative assess-ment3. The AFI assessment has since been shown to be areproducible method4–8, and a good correlation has beendemonstrated with dilution methods9,10 and with volumesaspirated during Cesarean section in late pregnancy11. In astudy by Magann and co-workers, the AFI predicted anormal quantity of amniotic fluid in 74% of 40 women in

late pregnancy and polyhydramnios in 83%, but a lowsensitivity was found in cases with oligohydramnios,defined as a volume of < 500 ml12. Dilution methodsare still considered the ‘gold standard’, although their reli-ability in cases of oligohydramnios is also questionable,since amniocentesis may be difficult when the quantity ofamniotic fluid is reduced and mixing of the diluent maybe less than adequate.

Normal values of the AFI from cross-sectional data havebeen reported7,13–16. All these studies show a gradual fall ofthe mean AFI during the last weeks of pregnancy and aconsiderable interindividual variation. The only study oflongitudinal design describing normal values of the AFIthroughout pregnancy was performed by Nwosu and associ-ates17. Their findings differ from cross-sectional observa-tions mainly in the definition of the lower levels ofnormality. Three studies have concentrated on the changesof the AFI occurring in pregnancies beyond 40 weeks’gestation3,18,19, and only the last two have addressedindividual patterns. Gestational age was determined by‘reliable dates’ in the first, and ‘reliable dates consistentwith ultrasound’ in the second and third studies, without

The Journal of Maternal–Fetal and Neonatal Medicine 2002;12:291– 297

Correspondence: Dr R. H. Stigter, University Medical Centre Utrecht, PO Box 85090, 3508 AB Utrecht, The Netherlands

OR IGI NAL AR T I CL E 291 Received 02–08–02 Accepted 21–08–02

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specification of the gestational age when ultrasonographywas first performed or the criteria used to confirm theaccuracy of the gestational age.

The current study was undertaken to determine thegestational age-related changes in AFI in term and post-term pregnancies, where gestational age was calculated onthe basis of a first-trimester crown–rump length (CRL)measurement, and to examine the relationship between theAFI and the time of onset of spontaneous labor. We alsoexamined the relationship between AFI and fetal outcome.

METHODS

This study was approved by the local ethics committee, andinformed consent was obtained from all participants.Women with uneventful pregnancies were recruited fromthe outpatient department at a mean gestational age of 275days (range 249–295 days). Measurements were performedat weekly intervals until 41 weeks and then twice weeklyuntil delivery. Details on the number of measurementsperformed in the study population are shown in Table 1.A total of 220 AFI measurements were performed in 137women. Gestational age had been determined by CRLmeasurement in 112 women and last menstrual period(LMP) in 20, and was considered uncertain in five women.Labor was induced in 31; 105 of the 137 women hadspontaneous onset of labor; the outcome was unknown inone. More than one AFI measurement was available from51 of the 137 women: two measurements in 30, three in 15,four in three, five in one and six measurements in twowomen. The AFI was measured as described by Phelan andco-workers3 with women in the supine position for a briefperiod. The abdomen was divided into four quadrants,the linea nigra separating the left from the right and theumbilicus the lower from the upper halves. The maximumpocket depth in each of the four quadrants was measuredwith the ultrasound transducer placed perpendicular to thefloor. The AFI was calculated as the sum of the fourmeasurements. All examinations were performed by asingle observer (R.H.S.) using a Toshiba Sonolayer

SSH140A (Toshiba Medical Systems Division, Tokyo,Japan) equipped with a 3.75-MHz curvilinear transducer.During the study period, it was local routine policy to assessthe amniotic fluid quantity by measuring the maximumpocket depth and to monitor fetal condition by thenon-stress test (NST) twice weekly after 290 days’ gesta-tion. Labor was induced on request after 41 completedweeks if the cervix was ripe, or earlier on finding abnormaltest results (reduced fetal heart rate (FHR) variation and/ordecelerations in a 60-min NST or a maximum pocket sizeof < 1 cm). Pregnancies were otherwise allowed to progressto a maximum of 43 weeks.

Severe fetal distress was defined as the presence of twoor more of the following conditions: a 5-min Apgar score of< 7; a venous cord pH of < 7.20; the need for resuscitationimmediately after delivery; admission to the special care orneonatal intensive care department; pediatric diagnosisof asphyxia or assisted delivery or Cesarean section for‘fetal distress’. Moderate distress was considered if only oneof the above was recorded, with or without meconiumstaining of the amniotic fluid.

Data are presented as means with standard deviations,and the AFI as D/SD values where D is the differencebetween the AFI measured and the normal mean for gesta-tional age, and SD the standard deviation of the distribu-tion of AFI measurements, based on the normal valuespresented by Nwosu and colleagues17. In cases where morethan one measurement was available, only the last AFImeasurement was selected for the purpose of cross-sectionalanalysis. Statistical analysis was performed by regressionanalysis and by the Kruskal–Wallis or Mann–Whitney test;significance was assumed at a p value of < 0.05 (two-tailed).

RESULTS

There was no significant relationship between the AFI andgestational age between 37 and 42 weeks in 109 indepen-dent cases in which gestational age was determined byfirst-trimester ultrasonography (R = 0.07) (Figure 1). The

Amniotic fluid index in late pregnancy Stigter et al.

J ournal of Maternal–Fetal and Neonatal Medicine

292

AFI measurements (n)made in each patient

Women examined(n)

Total number ofAFI measurements

Women with spontaneousonset of labor (n)

Women with accurategestational age (n)

123456

863015312

866045125

12

6826

92

––

712413

3–1

Total 137 220 105 112

Table 1 Details on the number of amniotic fluid index (AFI) measurements performed in the study population

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relationship between the AFI and the time of onset ofspontaneous labor is shown in Figure 2. The data were bestfitted by a polynomial regression line: AFI = 0.052 ´(number of days to delivery)2 -1.15 ´ (number of days todelivery) + 8.50. The curve yielded a maximum value by 11days before delivery with a reduction thereafter. Thisreduction in AFI from 11 days before the onset of labor wasexamined by linear regression and showed a significantlinear reduction (R = -0.37, n = 83, p < 0.001). Figure 3ashows the relationship between AFI and gestational age,and Figure 3b the relationship between AFI and onset oflabor in 88 patients with accurately dated pregnancies whoalso had spontaneous onset of labor. Oligohydramnios,defined as an AFI of < 5 cm, occurred as early as 265 days of

gestation, but was observed only during the last 4 daysbefore the onset of spontaneous labor.

The AFI appeared to remain within a narrow range overtime within each individual. This was examined by study-ing the correlation between AFI measurements withinindividuals at various time intervals. For the 51 patients,where more than one AFI measurement had been obtained(range 2–6), 130 pairs of AFI measurements could be made.By random selection of one pair of AFI measurements fromeach patient, 51 pairs of normalized AFI values were avail-able and analyzed by regression. In this subgroup, the meantime interval between pairs of AFI measurements was8 days (range 1–22 days) and the resulting regression linedid not deviate greatly from the line of equality (Figure 4).

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Figure 1 Relationship between gestational age and the amnioticfluid index (AFI) in 109 accurately dated pregnancies. Only thelast value of each case is shown

Figure 2 Relationship between amniotic fluid index (AFI) andthe onset of spontaneous labor (0 days). Only the last 3 weeksbefore delivery are shown

Figure 3 The amniotic fluid index (AFI) of 88 cases where gestational age was determined on the basis of a first-trimester ultrasoundscan and labor onset was also spontaneous. Horizontal lines indicate the commonly defined level of oligohydramnios; vertical linesindicate the time by which the first abnormal measurement occurred. (a) AFI measurements in relation to gestational age; (b) the samemeasurements in relation to the time of onset of labor

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Further groups with an interval of 1, 2 and more than 2weeks between AFI measurements were randomly selectedfrom the 130 pairs available, and the results are shown inTable 2, based on both absolute values of the AFI, and theircorresponding D/SD values. As can be seen from the table,a significant correlation was found between AFI measure-ments made within individuals separated by time periods ofup to 14 days.

The presence of meconium staining of the amnioticfluid was closely related to the gestational age at the time ofdelivery, and was observed only from 40 weeks’ gestationonward (Table 3). Meconium was present in 15% of casesdelivered at 40 weeks, 35% at 41 weeks and 28% at 42weeks. Meconium staining was not significantly related tofetal distress or to the quantity of amniotic fluid (Table 3).A degree of fetal compromise occurred in 15 of 47 caseswith certain gestational age where more than one AFImeasurement had been made. There was a trend towards apoorer outcome with lower last recorded AFI values and agreater reduction between the last two measurements, butthe differences were not statistically significant (Table 4).



294

Figure 4 Relationship between pairs of amniotic fluid index(AFI) measurements obtained at a mean time interval of 8 days(range 1–22 days) from each other. The AFI is shown as the D/SDon the basis of the normal values of Nwosu and colleagues (refer-ence 17). The regression line and the line of equality are shown

Time interval(range in days)

Mean interval(days)

AFI (cm) D/SD AFI

n R p Value R p Value

1–78–14

15–22

61018

3219

8

0.590.730.57

< 0.001< 0.001

NS

0.580.750.59

< 0.001< 0.001

NS

1–22 8 51 0.57 < 0.001 0.54 < 0.001

Table 2 The correlation between amniotic fluid index (AFI) measurements made on two occasions and at various time intervalswithin one individual. Data are grouped in 1-week intervals and presented as the regression coefficient R and the corresponding p valuesfor both absolute values of AFI and as values corrected for gestational age (D/SD AFI) on the basis of normal values given by Nwosu andcolleagues (reference 17). (n = number of pairs of AFI measurements)

GA atdelivery(weeks)

Fetal distress (n) AFI (cm) D/SD AFI

Meconium +Meconium

+Meconium

-

Meconium + Meconium -p

Value*

Meconium + Meconium -p

Value*n n % Mean SD Mean SD Mean SD Mean SD

39404142

10264029

04

148

0153528

0254

3355

—14.710.711.2

—6.24.55.2

—10.812.79.1

—4.35.33.8

—NSNSNS

—-0.4-1.4-0.4

—2.71.72.4

—-1.7-0.7-0.5

—1.82.22.3

—NSNSNS

All 105 26 25 11 16 11.4 4.8 10.9 4.7 NS -1.0 -1.1 NS

*Mann–Whitney test (two-tailed)

Table 3 The relationship between meconium staining (+, present; -, absent) of the amniotic fluid at delivery and gestational age(GA), fetal distress and the quantity of amniotic fluid. Data are presented as means and standard deviations (SD). For definition of fetaldistress, see text

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DISCUSSION

Like all previous studies, whether using dilution methods orultrasonographic techniques, we observed a small, butnon-significant reduction in the quantity of amniotic fluidin pregnancies progressing beyond term. In contrast tomany previous studies, all cases used in our study todescribe the relationship with gestational age had gesta-tional age calculated by ultrasonographic measurement ofthe fetal CRL during the first trimester. This is particularlyimportant since inadequate dating contributes significantlyto the population of ‘post-date pregnancies’20–22. The accu-racy of ultrasonographic measurements to assess gestationalage allows for an error of up to 7 days when the CRL hasbeen measured during the first trimester, but the un-certainty increases to almost 14 days when the biparietaldiameter (BPD) or femur length have been measuredduring the late second trimester23. We found a significantreduction of the AFI during the last 11 days before theonset of spontaneous labor. This indicates that the reduc-tion of the AFI is more closely related to the onset ofspontaneous labor than to the exact gestational age. Whencomparing the AFI measurements of 88 accurately datedpregnancies where onset of labor was spontaneous both inrelation to gestational age and in relation to onset of labor,it becomes clear that oligohydramnios (AFI < 5 cm)occurred only during the last 4 days before spontaneousonset of labor, although it can be seen as early as 38 weeksof gestation. This implies that oligohydramnios is usuallyfollowed by spontaneous onset of labor within days. Thereduction in the quantity of amniotic fluid at term can thusbe viewed as the result of a physiological process, coincid-ing with the onset of labor, or, alternatively, as a feature ofa pathological process where the fetus uses some rescuemechanism to initiate labor even as early as 38 weeks.

The association between a reduced quantity of amnioticfluid and unfavorable outcome is well established. Oligo-hydramnios, defined as a maximum amniotic fluid pocketsize of < 1 cm, was associated with intrauterine growthrestriction and increased perinatal mortality at any gesta-tional age in a large number of pregnancies reported byChamberlain and co-workers24. Crowley and associates25

used the deepest pocket of amniotic fluid to monitor pro-longed pregnancies, and found a significant relationshipbetween oligohydramnios and growth restriction, meco-nium staining of the amniotic fluid and fetal distress duringlabor. Our study shows a clear relationship between meco-nium staining of the amniotic fluid and gestational age,confirming previous findings by Steer and associates26, butnot with the AFI. In our study, the relationships betweenAFI measurements and fetal outcome were comparable toobservations by others18,19. Outcome was generally lessfavorable in association with lower mean AFI values anda greater mean reduction of the AFI between the lasttwo assessments, although these differences were not statis-tically significant.

We found considerable consistency of the AFI, both inabsolute terms and after correction for gestational age,which has previously been observed by Geirsson in hisultrasound studies on total intrauterine volume2, and byTomoda and co-workers in a fetal sheep model27.

Little is known about the apparently precise regulationof the amniotic fluid volume. The most important contri-bution is formed by fetal urine production and secretion offluid from the respiratory tract. Fluid is removed by fetalswallowing and possibly by absorption through fetal mem-branes. A vascular plexus has been described in the ovineplacenta capable of the net removal of water from theamniotic compartment by Brace and colleagues andthe existence of ‘intramembranous absorption’ of water has

295



No fetal distress(n = 32)

Mild fetal distress(n = 8)

Severe fetal distress(n = 7)

Mean SD Mean SD Mean SD p Value*

Gestational age at last examination (days)Mean interval between AFI measurements (days)Interval between final test and delivery (days)AFI penultimate examination (cm)AFI final examination (cm)Reduction in AFI of last two measurements (%)Gestational age at delivery (days)Birth weight (g)

2876.42.9

11.411.5

4.6290

3605

62.51.84.75.5

40.56.2

383

2866.93.99.69.3

-2.5290

3783

72.73.33.54.3

32.55.9

688

2866.93.4

12.38.7

-18.3290

3685

34.02.16.94.6

51.32.1

734

NSNSNSNSNSNSNSNS

*Comparison between groups made by the Kruskal–Wallis test; NS, not significant

Table 4 Relationship between amniotic fluid index (AFI) measured during the last two examinations before labor in 47 of the 51individuals where at least two measurements were made and outcome data were available

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been demonstrated in the rhesus monkey by the same groupof researchers28,29. However, there is no evidence of thepresence of a comparable vascular network in human fetalmembranes30.

The changes occurring in post-term pregnancy haveoften been considered as a form of placental insufficiency.In the growth-restricted fetus, a redistribution of the fetalcirculation has been described whereby the central nervoussystem, adrenals and coronary circulation are spared at theexpense of less vital organs, including the kidneys. Fetalurine production was found to be reduced in the small-for-gestational-age fetus31–33. Studies on fetal urine produc-tion in term and post-term pregnancies, however, producedconflicting results, with some studies reporting a reduc-tion33–35, an increase36, or no change37. Arduini and Rizzofound increased pulsatility index values in the fetal renalartery in growth-restricted fetuses, particularly whenoligohydramnios was present, but did not find such arelationship in post-term pregnancies38. Bar-Hava andco-workers also failed to find changes in the fetal circu-lation in relation to oligohydramnios in pregnanciesbeyond term39.

We found changes in the fetal circulation in relation tothe time of onset of labor, rather than gestational age, con-sistent with a subtle redistribution in the group of patientsdescribed in this study. We also observed reduced fetalurine production in relation to time of onset of labor. Thesefindings, combined, suggest that placental function indeedbecomes increasingly ‘insufficient’ at some stage near term.Through a mechanism of circulatory redistribution andreduced fetal urine production, a reduction of amnioticfluid volume results, which is usually followed by the spon-taneous onset of labor.

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the amniotic fluid index in late pregnancy

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