Screening for fetal growth disorders by clinical exam in the era of obesity

OBJECTIVE: To evaluate the performance of clinical estimation of fetal weight as a screening test for fetal growth disorders and then to estimate the effect of maternal body mass index (BMI) on its screening efficiency. STUDY DESIGN: This was a retrospective cohort study of patients referred for third trimester ultrasound for the indication of “size unequal to dates”. Patients with medical co-morbidities which may alter their a priori risk for fetal growth disorders were excluded. The incidence of fetal growth disorders as well as amniotic fluid disturbances was determined for each group and then compared across maternal BMI categories of <25 kg/m(2), 25-30 kg/m(2), ≥30 kg/m(2), and ≥40 kg/m(2). To evaluate the accuracy of clinical estimation of fetal weight in predicting fetal growth disorders, the sensitivity, specificity, positive predictive value, negative predictive value, likelihood ratios, as well as number needed to scan (NNS) was calculated and compared across BMI categories. RESULTS: Of 51,366 patients, 1,623 were referred for the indication of size > dates and 1,543 for the indication of size < dates. The incidence of fetal growth disorders in each referral group was low and was not significantly different across BMI categories. The sensitivity and specificity were 9.7% and 96.6% for predicting neonatal birth weight (BW) >90(th)%ile and 13.5% and 96.7% for predicting BW <10(th)%ile. The NNS to detect one neonate with a BW <10(th)%ile ranged from 5-19 while the NNS to detect one neonate with a BW >90(th)%ile ranged from 6-13 across BMI categories. CONCLUSION: Overall, clinical estimation of fetal weight yields a low detection rate of fetal growth abnormalities; however, its screening efficiency is not adversely impacted by maternal BMI.