Receiver operating characteristic curves of ultrasonographic estimates of fetal weight for prediction of fetal growth restriction in prolonged pregnancies.

OBJECTIVE: Recent studies have documented increased perinatal morbidity and mortality rates in the growth-restricted postterm fetus. Our purpose was to evaluate the receiver operating characteristic curve of ultrasonographically estimated fetal weight as a predictor of fetal growth restriction in prolonged pregnancies. STUDY DESIGN: Fetal weight was estimated ultrasonographically within 9 days of delivery (mode 1 day) in members of a cohort of 410 patients with prolonged pregnancies (>41 weeks). Estimated fetal weights were compared with birth weights in receiver operating characteristic curve analysis. RESULTS: The areas under the receiver operating characteristic curves for predicting birth weights <10th percentile (3125 g in this population) and <5th percentile (2930 g in this population) were 0.89 and 0.96, respectively. Both areas were significantly different from an area indicating a useless test. The estimated fetal weight values corresponding to the inflection points for the receiver operating characteristic curves predicting birth weights <10th percentile and <5th percentile were 3370 and 3200 g, respectively. With estimated fetal weight at less than these test cutoff values, the relative risks for a fetus to have a birth weight <10th percentile or <5th percentile were 14.6 (95% confidence interval, 6.25-33.8) and 89.8 (95% confidence interval, 12.1-665), respectively. Analysis of the receiver operating characteristic curves resulted in improved test characteristics relative to using the actual 10th and 5th birth weight percentiles as cutoff values for estimated fetal weight (relative risk of 14.6 vs 9.5 and 89.8 vs 26.0, respectively). CONCLUSIONS: Ultrasonographic estimation of fetal weight is a useful test for predicting fetal growth restriction in prolonged pregnancies. Future studies should evaluate whether intervention on the basis of this identification results in improved perinatal outcome.

Receiver operating characteristic curves of sonographic estimated fetal weight for prediction of macrosomia in prolonged pregnancies.

We evaluated areas under receiver operating characteristic curves for sonographic estimated fetal weight (EFW) as a predictor of macrosomia in prolonged pregnancies. These areas were 0.85 for birth weights of > or = 4000 g and 0.95 for birth weights of > or = 4500 g. Both were significantly greater than 0.5, the area under curves for useless tests. Areas under curves before and after adjustment for time elapsed between measurement and delivery did not differ significantly. At the inflexion point cut-off level of 3711 g, sensitivity, specificity and positive and negative predictive values of EFW for birth weight of > or = 4000 g were 85, 72, 49 and 94%, respectively. At the inflexion point cut-off level of 4192 g for birth weight of > or = 4500 g, these values were 83, 92, 30 and 99%. The relative risk for birth weight of > or = 4000 g was 7.99, and for birth weight of > or = 4500 g, 39.50, both significant. In conclusion, sonographic EFW is a useful test for predicting macrosomia. Adjustment for time elapsed did not significantly improve either test, probably because of slow rates of fetal growth at this gestational age. Cut-off values derived from this analysis result in high sensitivity but low positive predictive value. A randomized controlled trial of mode and timing of delivery for predicted macrosomia is needed.

Predictive value of amniotic fluid index for oligohydramnios in patients with prolonged pregnancies.

The objective of this study was to evaluate the predictive values of the amniotic fluid index for measures of perinatal morbidity and for clinical observations consistent with oligohydramnios. We evaluated positive and negative predictive value of the amniotic fluid index for measures of perinatal morbidity and for clinical observations consistent with oligohydramnios at various cutoff values for amniotic fluid index in a cohort of 449 consecutive postdates patients who had a clinician's observation of amniotic fluid quantity and quality recorded at the time of rupture of membranes. Newborn morbidity was a rare event. Clinical observations consistent with oligohydramnios had significant positive and negative predictive values for some measures of newborn morbidity. The last amniotic fluid index performed during antepartum testing had 95% confidence intervals for relative risks for these measures of newborn morbidity that included unity and therefore were not significant. At a cutoff value of 5.0 cm, the positive predictive value of the amniotic fluid index for clinical observations consistent with oligohydramnios was 50%; the negative predictive value was 85%, with a prevalence of clinical observations consistent with oligohydramnios of 19%. The presence of fetal heart rate decelerations did not significantly improve the positive predictive value of the amniotic fluid index. Higher positive predictive values were obtained at cutoff values of 4 cm and 3 cm with minimal loss in negative predictive value. The amniotic fluid index did not possess significant predictive value for measures of newborn morbidity. Clinical observations consistent with oligohydramnios at the time of rupture of membranes did have predictive value for some of these measures and thus probably are a reflection of the actual amount of fluid present inside the uterus prior to rupture of membranes. The amniotic fluid index is only a fair predictor of clinical observations consistent with oligohydramnios. Thus, a positive test correctly predicted these observations 50% of the time, with 50% false-positive results. Undertaking delivery in the 50% of patients without clinical observations consistent with oligohydramnios may lead to a higher cesarean section rate since these patients do not require induction and are subject to the risk of a failed induction of labor. A negative test correctly predicted observations consistent with normal fluid 85% of the time, with a false-negative rate of 15%. Thus, a negative test was no guarantee that observations consistent with oligohydramnios, and thus newborn morbidity, would not subsequently appear. Frequent testing with multiple modalities and induction of labor when the Bishop score is favorable remain sensible options. Induction of labor in postdates patients with a low amniotic fluid index needs to be evaluated in a yet-to-be-performed prospective randomized control trial before a low amniotic fluid index is assumed to be the sole indicator for induction of labor. More stringent cutoff values for amniotic fluid index may be justified.