Four-quadrant assessment of amniotic fluid volume. Interobserver and intraobserver variation

While the use of the four-quadrant technique, or the amniotic fluid index (AFI), for amniotic fluid volume assessment has been shown to correlate with perinatal outcome, one concern among clinicians is the reliability of a single AFI measurement. The purpose of this investigation was to ascertain the margin of error using this technique among 23 term and postterm women. The AFI was obtained using linear array real-time B-scan ultrasound to measure the vertical diameter of the largest amniotic fluid pocket in each of the four quadrants. The sum of these measurements was expressed as the AFI. Eight women underwent ten consecutive AFI measurements by the same sonographer to determine intraobserver variation. Interobserver variation between five sonographers was measured by assessing the AFI in 15 additional patients. With the amniotic fluid volume in the low or normal range, our results demonstrated the intraobserver and interobserver variations in AFI to average 1.0 and 2.0 cm, respectively. With an above-normal amniotic fluid volume, a 2.5- to 3-fold greater variation was observed. A relatively small margin of error appears to exist between observers using the four-quadrant technique to assess amniotic fluid volume.     

The role of amniotic fluid assessment in evaluating fetal well-being

Owing to the frequent use of bedside ultrasound, much is known about the regulation of and normative values for amniotic fluid volume and the mechanisms by which this fluid is regulated. The management protocols for conditions with extremes of amniotic fluid volume have become more exact, resulting in interventions more likely to improve outcome. Much is still unclear; there are no tools to measure amniotic fluid volume with precision, and measurement of fetal urinary output is cumbersome and error-prone.     

Use of the drop volume of amniotic fluid in predicting fetal lung maturity. Clinical experience

Respiratory distress syndrome (RDS), resulting from inadequate amounts of alveolar surfactant, is a leading cause of neonatal mortality. This study analyzed the drop volume for predicting the risk of neonatal RDS by evaluating the surface tension of amniotic fluid. The drop-volume method quantifies the surface tension of amniotic fluid using the fact that the volume of a falling drop of liquid is proportional to the quantity of surface-active substances (surfactant) in the solution. Distilled water is used as a reference solution. The test is simple to perform and requires only two minutes and 2 mL of amniotic fluid. Amniotic fluid was obtained by amniocentesis from 120 pregnancies, ranging from 24 to 42 weeks of gestation, within four days of delivery. Specimens contaminated with meconium or hemolyzed blood or from patients with clinical chorioamnionitis were not used. The lecithin:sphingomyelin ratio and phosphatidylglycerol tests were also determined for 54 of the study cases. A mature drop-volume test result was associated with a healthy infant with no RDS 95% of the time (negative predictive value). Fifty-two percent of the healthy infants in the study were predicted by an arbitrary unit of time (S) value of less than or equal to 85.0 (specificity). Thirteen of the 16 infants who developed RDS (81%) were predicted by an S value of greater than or equal to 85.1 (sensitivity). Of those infants with an S value of greater than or equal to 85.1, 21% developed RDS (positive predictive value).     

Ultrasound evaluation of amniotic fluid volume. II. The relationship of increased amniotic fluid volume to perinatal outcome

Qualitative amniotic fluid volume determination is a routine part of fetal biophysical profile score testing. The relationship between polyhydramnios and poor perinatal outcome has been previously documented. We have undertaken a retrospective chart review which relates qualitative amniotic fluid volume as determined at the time of last biophysical profile score assessment to perinatal outcome in 7562 referred high-risk obstetric patients. Gross and corrected perinatal mortality in association with normal qualitative amniotic fluid volume ranged from 4.65/1000 and 1.97/1000, respectively, to 32.9/1000 and 4.12/1000 in association with increased qualitative amniotic fluid volume, respectively. The incidences of major congenital anomaly and fetal macrosomia were significantly related to qualitative amniotic fluid volume.     

The fetal outcome in pregnancies with isolated reduced amniotic fluid volume in the third trimester

Our aim was to assess the outcome of pregnancies where oligohydramnios, defined by a published gestational reference range for amniotic fluid index, was the only abnormal finding at third trimester scan, and all other ultrasound parameters including biometry were within normal limits at initial scan. A retrospective case-control study was performed at The Liverpool Maternity Hospital. 103 pregnancies with reduced amniotic fluid index in the third trimester and apparently normal fetal growth profile ultrasonographically were identified from ultrasound reports throughout 1993. Pregnancies in the third trimester with normal amniotic fluid index on index scan were also identified from these reports and 103 were matched for parity, gestational age at delivery, mode of onset of labour, presentation at labour and medical conditions. Exclusion criteria were ruptured membranes, fetal abnormalities, estimated fetal weight below the fifth centile at index scan and multiple pregnancies. The outcome criteria were birthweight, Apgar scores at delivery, induction and emergency delivery for fetal reasons and admission to Neonatal Intensive Care Unit. Statistical analysis was performed by Fisher's exact test and Gart's odds ratio. Compared with controls, pregnancies in the reduced liquor group had a higher number of babies below the 5th centile (odds ratio 5.2, 95% confidence interval 1.6 to 22), a higher risk of induction for fetal reasons (odds ratio 34.4, 95% confidence interval 5.35 to 1425.5) and admission to Neonatal Intensive Care Unit (odds ratio 9.77, 95% confidence interval 1.3 to 432). Any observed difference in the need for emergency delivery due to fetal reasons was not clinically significant (odds ratio 2.16, 95% confidence interval 0.77 to 6.6) The definition used for oligohydramnios used in this study appears to identify a group of babies with a fourfold risk of low birthweight and a high risk of admission to the Neonatal Intensive Care Unit and induction of labour for fetal reasons. This would suggest that pregnancies with isolated oligohydramnios require some form of fetal monitoring and further prospective studies are required to determine the most appropriate method.     

Dye-determined amniotic fluid volume and intrapartum/neonatal outcome.

OBJECTIVE: To ascertain if a dye-determined amniotic fluid volume was predictive of intrapartum and perinatal outcome. MATERIALS AND METHODS: The low and normal amniotic fluid volumes (< 5th percentile and > or =5th percentile for gestational age) and the raw dye-determined amniotic fluid distributions were correlated with 10 clinical outcome measures in 74 pregnancies. RESULTS: In this observational study, median gestational age at delivery was 36 weeks (range 26 to 41) and 16 deliveries were for fetal distress (14 Cesarean and two forceps). There were no differences between the outcomes of pregnancies with low and normal amniotic fluid volumes for any of the clinical outcomes (variable decelerations influencing delivery, p=0.381; late decelerations, p=0.875; Cesarean births for fetal intolerance of labor, p=0.259; intrauterine growth restriction, p=0.998; or umbilical cord arterial pH< 7.2, p=0.259). Analogous results were obtained when the gestational age-adjusted amniotic fluid volumes were compared directly between the pregnancies with normal and abnormal outcomes. There was no difference between the mean amniotic fluid volumes in those pregnancies with variable decelerations influencing delivery (p=0.287), late decelerations (p=0.555), Cesarean births for fetal intolerance of labor (p=0.310), intrauterine growth restriction (p=0.267) or umbilical cord arterial pH< 7.2, and the pregnancies without these intrapartum events. Reduced variability was more commonly observed in pregnancies with higher amniotic fluid volumes (p=0.038, 771 ml, 95% CI 468 to 1269, compared to those without normal variability 444 ml, 95% CI 374 to 526). CONCLUSIONS: Dye-determined amniotic fluid volume does not appear to be predictive of adverse intrapartum and neonatal outcome.     

Pathological pregnancies. Results of amniotic fluid studies and fetal outcome

Late amniocenteses (greater than 20 weeks' gestation) were performed in 114 pregnancies with no a priori genetic risk, but referred because of abnormal clinical and/or ultrasound findings suggesting fetal malformations. Reasons for referral included polyhydramnios (51 cases), oligohydramnios (15 cases), fetal growth retardation (FGR) (16 cases) and abnormal fetal ultrasound findings excluding anencephaly (32 cases). In 42 of these cases, referral was motivated by a combination of the above abnormal findings. When polyhydramnios was the sole anomaly (25 cases), 5 fetuses were malformed (20%), abnormal fetal karyotype and/or elevated amniotic fluid alphafetoprotein (AFP) were demonstrated in 2 cases. Oligohydramnios was the sole anomaly in one case; the infant died of prematurity. Fetal growth retardation was the sole anomaly in 14 cases, 11 otherwise normal newborns were small for date, 2 died at birth and 1 was malformed (1/14, 7%). In this group all fetal karyotypes were normal and in 2 cases amniotic fluid AFP were increased. In the 32 pregnancies without abnormal amniotic fluid volume and/or FGR and with fetal malformation(s) suggested by ultrasound, all malformations except one (ovarian cyst possibly ruptured during birth) were confirmed at birth, amniotic fluid AFP was elevated, and/or karyotype was abnormal in 6 cases. In 42 pregnancies where more than one alarm sign was present, abnormal karyotype and/or elevated amniotic fluid AFP level were recorded in 21 of the 39 cases where amniocentesis was performed, 33 fetuses were malformed (79%) and 13 died in the perinatal period (31%). The high incidence of abnormal results of amniocentesis found in this survey of pathological pregnancies, particularly in those with multiple alarm signs, emphasizes the need for amniocentesis in these situations.     

Nonstress testing with acoustic stimulation and amniotic fluid volume assessment: 5973 tests without unexpected fetal death

In a 36-month period antepartum testing was performed 5973 times in 2628 women with singleton high-risk pregnancies. The testing scheme involved a modified nonstress test with sound stimulation. Testing was performed twice weekly for patients with diabetes (classes B through R), gestational age exceeding 42 weeks, and documented intrauterine growth retardation, and weekly for other indications. If no spontaneous acceleration was observed within 5 minutes, a single 1- to 2-second sound stimulus was applied to the lower maternal abdomen with an artificial larynx. If necessary, a second sound stimulation was applied within 10 minutes. In addition all patients received ultrasonographic four-quadrant assessment of amniotic fluid volume. The mean testing time was 10 minutes. Only 2% of tests were nonreactive with sound stimulation. Seventeen percent of nonstress tests that were nonreactive with sound stimulation were followed by positive results of a contraction stress test or a biophysical profile score less than or equal to 4. The overall intervention rate was 3%. All fetuses with a single acceleration only eventually met criteria for negative results to a contraction stress test or had a biophysical profile score greater than or equal to 8. There were no unexpected antepartum fetal deaths. Sound-induced accelerations appear to be valid in the prediction of fetal well being, and the use of sound stimulation results in a significant shortening of testing time. Simultaneous assessment of amniotic fluid volume may reduce the risk of fetal death to a negligible level.     

Regulation of amniotic fluid volume: intramembranous solute and volume fluxes in late gestation fetal sheep

OBJECTIVE: Recent studies suggest that amniotic fluid volume is regulated by the rate of intramembranous absorption of amniotic fluid into fetal blood. The purpose of the present study was to determine the simultaneous intramembranous solute and water fluxes to gain insight into the intramembranous transport and amniotic fluid volume regulatory mechanisms. STUDY DESIGN: All major amniotic inflows and outflows, except intramembranous flow, were eliminated in 10 fetal sheep over 8 hours by occlusion of the fetal trachea and esophagus; the fetal urine was drained to the exterior. Amniotic fluid composition and volume were measured before and at the end of the 8 hours. Solute and volume fluxes through the intramembranous pathway were calculated from amniotic fluid concentration and volume changes. Statistical analyses included t-tests, linear regression, and analyses of variance. RESULTS: Amniotic fluid volume decreased by 128 +/- 24 (SE) mL over 8 hours (P < .001), which was correlated only marginally with the fetal to amniotic fluid osmotic gradient (r=0.59; P = .072). Amniotic fluid sodium, chloride, calcium, and bicarbonate concentrations increased (P < .0001), even though there were net outward fluxes of these solutes; these outward fluxes occurred against concentration gradients; and the clearances of these solutes were the same despite widely differing amniotic fluid concentrations and fetal blood to amniotic fluid concentration gradients. With the use of multivariate regression, intramembranous solute fluxes separated into 2 components, which were a primary outward flux that correlated with the volume flux and a minor inward component that correlated with the fetal plasma to amniotic fluid concentration gradient for sodium, chloride, calcium (P < .001), and bicarbonate (P < .02). The concentration-dependent fluxes averaged approximately one third of the bulk fluxes and were in the opposite direction. CONCLUSION: The poor correlation of amniotic fluid volume reduction with the fetal-to-amniotic fluid osmotic gradient shows that the primary mechanism that mediates intramembranous volume flow is not passive osmosis in the normal fetus under basal conditions. The strong correlations of solute fluxes simultaneously with volume flux and concentration gradients suggest that intramembranous solute fluxes are mediated by both bulk flow and passive diffusion. The small size of the passive component relative to the size of the bulk component suggests that intramembranous solute transfer is mediated primarily by bulk flow with a smaller and usually oppositely directed contribution by diffusion down concentration gradients. Bulk flow by vesicular transport is the only known physiologic transport mechanism that is compatible with these data, but it is not known whether this occurs in the amnion or intramembranous blood vessels or both.     

Ultrasound evaluation of amniotic fluid volume. I. The relationship of marginal and decreased amniotic fluid volumes to perinatal outcome

Qualitative amniotic fluid volume determination is a routine part of fetal biophysical profile score testing. The relationship between oligohydramnios and poor perinatal outcome has been previously documented. We have undertaken a retrospective chart review relating qualitative amniotic fluid volume as determined at the time of last biophysical profile score assessment to perinatal outcome in 7582 referred high-risk obstetric patients. Gross and corrected perinatal mortality in association with normal qualitative amniotic fluid volume ranged from 4.65/1000 and 1.97/1000, respectively, to 187.5/1000 and 109.4/1000 in association with decreased qualitative amniotic fluid volume, respectively. The incidences of major congenital anomaly and intrauterine growth retardation were significantly related to qualitative amniotic fluid volume.     

Amniotic fluid index: correlation with amniotic fluid volume.

We calculated the amniotic fluid indexes (AFIs) of 310 women on 459 occasions. Normative data were analyzed and compared with data in several high-risk groups. In the normal gestations there was a progressive increase in AFI with advancing gestation until 32 weeks, after which there was a decline. The mean AFIs in abnormal gestations varied with the clinical diagnoses. These values were compared to those obtained by assessing amniotic fluid volume (AFV), that is a pocket more than 2 cm. There were 51 patients with abnormal AFVs. Forty-two had decreased fluid, six also had decreased AFIs; nine had increased AFVs and five (all with diabetes) also had increased AFIs. Thus, AFIs in normal pregnancies showed an orderly pattern of change with gestational age, and there was no accurate correlation between AFI and AFV. Thus, using AFV alone may lead to false interpretations of amniotic fluid status.     

The relationship of increased amniotic fluid volume to perinatal outcome

We studied a group of 135 patients with polyhydramnios diagnosed on the basis of ultrasonic findings (greatest vertical diameter of the liquor column greater than or equal to 8.0 cm) between 32 and 36 weeks gestation (study group). We compared the obstetric characteristics and perinatal outcome of the study group with a similar number who constituted our control group. The incidence of women aged 20 years or less was higher in the study group (8.9%) compared with 4.5% in the control group. Of the 135 patients who were diagnosed to have polyhydramnios ultrasonically, the clinical diagnoses prior to referral for ultrasonic scanning, were, suspected large for date fetuses in 34 patients (25.2%), clinically suspected polyhydramnios in 28 (20.7%), gestational diabetes in 21 (15.6%) and insulin dependent diabetes in 6 (4.4%) compared with 13.3%, 5.2%, 3.0% and 0.7%, respectively in the control group (P less than 0.05). Preterm delivery occurred in 11.1% in the study group compared with the incidence of 6.7% in the control group. The incidence of fetal distress, low Apgar Score, macrosomic infants, major fetal anomalies, gross and corrected perinatal mortality rate and admission to special/intensive care nursery was significantly higher in the study group compared with that of the control (P less than 0.01). We found ultrasonic examination is a reliable technique to assess the amount of amniotic fluid volume and it alerts the clinician to possible future problems in pregnancy, labor and neonatal period.     

The role of ultrasound assessment of amniotic fluid volume in the management of the postdate pregnancy

Antepartum assessments of amniotic fluid volumes and their relationship to nonstress test patterns and pregnancy outcomes were retrospectively analyzed in 234 postdate pregnancies. The incidence of clinical oligohydramnios and a nonstress test revealing fetal heart rate deceleration or bradycardia was found to increase as the sonographic estimates of the amniotic fluid volume decreased. Furthermore, the postdate pregnancy with sonographic evidence of an adequate amniotic fluid volume had a significantly better perinatal outcome than the pregnancy without an adequate fluid volume. These results suggest that the postdate pregnancy with evidence of reduced amniotic fluid volume should be considered for a trial of labor with continuous electronic fetal monitoring.     

The usefulness of ultrasound assessment of amniotic fluid in predicting adverse outcome in prolonged pregnancy: a prospective blinded observational study

Objective To determine whether a single ultrasound scan at or beyond 40 weeks of gestation to detect a single deepest pool of amniotic fluid  <2 cm  and amniotic fluid index (AFI)  <5 cm  is clinically useful in the prediction of subsequent adverse pregnancy outcome.
Design A prospective double blind cohort study.
Setting A university teaching hospital delivering approximately 6000 women annually.
Population One thousand and five hundred and eighty-four pregnant women at or beyond 40 weeks of gestation.
Methods Ultrasound assessment of liquor to detect the single deepest pool of amniotic fluid and derive the AFI at or after 40 weeks of gestation.
Main outcome measures Perinatal death, meconium aspiration, birth asphyxia, intervention in labour for fetal distress, a cord arterial  pH <7  and admission to the neonatal unit.
Results An  AFI <5 cm  but not a single deepest pool  <2 cm  was significantly associated with birth asphyxia or meconium aspiration. An  AFI <5 cm  was also significantly associated with caesarean section for fetal distress in labour, a cord arterial  pH <7  at delivery and low Apgar scores. Despite there being a statistically significant association with adverse outcomes the sensitivity of AFI was low at 28.6%, 12% and 11.5% for major adverse outcome, fetal distress in labour or admission to the neonatal unit, respectively.
Conclusions The AFI is superior to a measure of the single deepest pool as an assessment of the fetus at or after 40 weeks but has a poor sensitivity for adverse pregnancy outcome. Routine use is likely to lead to increased obstetric intervention without improvement in perinatal outcomes.     

The significance of amniotic fluid volume during intrapartum fetal acoustic stimulation

The relationship of intrapartum fluid volume to the fetal response to acoustic stimulation was investigated in 112 patients who were in the latent phase of labor. Amniotic fluid volume appears to play an insignificant role in the genesis of fetal heart rate decelerations that occur after fetal acoustic stimulation, regardless of the amount of amniotic fluid present.     

The association of second trimester biomarkers in amniotic fluid and fetal outcome

Objective: To identify the level of amniotic fluid lactate (AFL), placental growth factor (PLGF), and vascular endothelial growth factor (VEGF) at second trimester amniocentesis, and to compare levels in normal pregnancies with pregnancies ending in a miscarriage, an intrauterine growth restricted fetus (IUGR) or decreased fetal movements.

Study design: A prospective cohort study. Amniotic fluid was consecutively collected at amniocentesis in 106 pregnancies. Fetal wellbeing at delivery was evaluated from medical files and compared with the levels of AFL, VEGF, and PLGF at the time of amniocentesis.

Results: The median level of AFL was 6.9?mmol/l, VEGF 0.088?pg/ml, and PLGF 0.208?pg/ml. The median levels of AFL in pregnancies ended in miscarriage were significantly higher (10.7?mmol/l) compared to those with a live new-born (6.9?mmol/L, p?=?.02). The levels of VEGF (p?=?.2) and PLGF (p?=?.7) were not affected. In pregnancies with an IUGR, the median level of AFL was higher compared to those with normal fetal growth (p?=?.003). No differences VEGF (p?=?.5), but significant lower PLGF were found in IUGR pregnancies (p?=?.03).

Conclusions: Pregnancies ending in a miscarriage or with IUGR had significantly higher median values of AFL but lower values of PLGF in the amniotic fluid at the time of second trimester amniocentesis compared to normal pregnancies.     

The maximal vertical pocket and amniotic fluid index in predicting fetal distress in prolonged pregnancy.

OBJECTIVE: To determine cut-off values for amniotic fluid volume (AFV) predictive of fetal distress in pregnancies 40 weeks or longer, assessed using both amniotic fluid index (AFI) and height of the of maximal vertical pocket (MVP). METHODS: Amniotic fluid index and MVP were evaluated in 100 women with pregnancies of 40 or more weeks and intact membranes using a 3.5-MHz linear transducer less than 1 week prior to onset of labor. Both measurements were obtained for each participant by the same obstetrician in one sitting. Oligohydramnios was defined as an AFI of 5 cm or less or an MVP of 3 cm or less. External cardiotocography was performed during intrapartum in all cases. Fetal distress was diagnosed when any one of the nonreassuring fetal heart rate pattern occurred or when the Apgar score at birth was 6 or less. The results were analyzed by the chi(2) and the t tests. RESULTS: An AFI of 5 cm or less was a better predictor of fetal distress than an MVP of 3 cm or less (P=0.000). The most statistically significant cut-off values for the occurrence of fetal distress in prolonged pregnancies were an AFI of 8 cm or less (odds ratio, 7.50) or an MVP of 2 cm or less (odds ratio, 2.67). There was a significant correlation between these 2 methods (r=+0.67), with a level of significance of 0.01, as shown by the regression curve. The secondary outcome was the statistically significant association between meconium-stained liquor and fetal distress (P=0.001). CONCLUSION: In pregnancies of 40 weeks or longer there was a risk of fetal distress when the AFV was 8 cm or less by the AFI method or 2 cm or less as measured by MVP. In such cases, intensive intrapartum monitoring should be performed to prevent fetal jeopardy.     

Reevaluation of the relationship between amniotic fluid volume and perinatal outcome

OBJECTIVE: Ultrasound estimation of amniotic fluid volume (AFI) is a critical component of antenatal surveillance. Alterations in AFI have classically been considered an indication of fetal compromise, but recent studies have called this into question. The present study was undertaken to reevaluate the relationship between AFI and perinatal outcome. STUDY DESIGN: The perinatal data base of the authors' institution was queried for all patients in a 6-year period who had AFI evaluated. Two groups, 1 high-risk and 1 low-risk, were evaluated to determine the relationship between AFI and gestational age. An additional high-risk group that had AFI determined within 48 hours of delivery was also used to correlate AFI with intrapartum and perinatal outcomes. Statistical analysis was performed using linear regression analysis to evaluate the correlation between AFI and gestational age or birth weight. Additional correlations were done using analysis of variance, chi-square, Fisher exact test, or Student t test. Interaction between variables was analyses using logistic regression analysis. RESULTS: Fourteen thousand seven hundred forty-seven AFI determinations in 4337 high-risk patients, and 1153 AFI determinations in 1153 low-risk patients were evaluated. There were no clinically significant correlations between AFI and gestational age. In the second high-risk group of 454 patients there was a significant correlation between polyhydramnios and large-for-gestational age infants, congenital anomalies, and an increase in cesarean section for delivery. There was an increased risk of nonreassuring fetal heart rate patterns during labor for the oligohydramnios patient, but only in preterm patients. There was no strong relationship between AFI and neonatal complications or length of stay in the neonatal intensive care unit. Logistic regression confirmed that AFI was not significantly correlated with perinatal outcome. CONCLUSION: The present study suggests that AFI is a weaker predictor of perinatal outcome than has been classically suggested. Although the AFI identification of polyhydramnios was helpful in identifying LGA fetuses and fetuses at risk for congenital abnormalities, oligohydramnios was a rather weak predictor of poor perinatal outcome.     

Correlation of subjective assessment of amniotic fluid with amniotic fluid index.

We assessed the correlation between abnormal amniotic fluid volumes as defined by the two techniques of (1) subjective evaluation and (2) the amniotic fluid index. Ultrasound evaluation of amniotic fluid volume was conducted on 420 pregnant women with known gestational age greater than twenty weeks but less than 42 weeks. Amniotic fluid was evaluated subjectively and placed into one of three categories: normal, oligohydramnios or polyhydramnios. After fetal biometry was performed, the amniotic fluid volume was assessed semi-quantitatively by the amniotic fluid index technique and assigned to similar categories. We analyzed the data with 2 x 2 contingency tables, using amniotic fluid index as the 'gold standard test'. Our study demonstrates that there was moderate agreement (kappa.5) between both amniotic fluid techniques in the identification of oligohydramnios. However, agreement between the techniques was poor for the identification of polyhydramnios (kappa.16).     

The role of amniotic fluid assessment in indicated preterm delivery

Measuring amniotic fluid pockets with ultrasound is an efficient and reasonably reliable method of evaluating amniotic fluid volume and categorizing relative risk of perinatal morbidity. The most commonly used ultrasound criteria for oligohydramnios, SDP <2 cm and AFI <5 cm, assign approximately 2%-3% and 4%-5% of late preterm pregnancies into the "low amniotic fluid" category. The AFI offers somewhat greater sensitivity and greater precision but has less specificity for predicting perinatal morbidity than does the SDP. Thus, before 34 weeks, use of the AFI <5 cm as a criterion for intensive fetal monitoring, but not as sole criteria for delivery, is recommended. In pregnancies beyond 34 weeks, use of either AFI or SDP to diagnose oligohydramnios can be expected to reliably identify fetuses at risk for compromised perinatal outcome especially if replicate measurements are confirmatory. In such cases, care must be taken to identify comorbid conditions that, together with oligohydramnios, may place the fetus at significant risk. In such cases, delivery is the recommended intervention.