LAS VEGAS – Fetal macrosomia can be challenging to detect by ultrasound performed just before delivery, which had 41% sensitivity and 58% positive predictive value in a prospective study of more than 2,300 pregnancies.
The results also showed that fetal macrosomia (defined as birth weight of more than 4,000 grams) is significantly linked with increased rates of prolonged labor, delivery by either operative vaginal or cesarean approaches, and postpartum hemorrhage, Daniel M. Galvin, MD, said at the annual Pregnancy Meeting sponsored by the Society for Maternal-Fetal Medicine.
Because all clinicians involved with these deliveries were blinded to the prenatal ultrasound results, the findings suggest that prolonged labor, postpartum hemorrhage, and need for either operative vaginal delivery or cesarean delivery are all outcomes driven by macrosomia itself rather than by clinical actions taken because of an expectation of macrosomia, said Dr. Galvin, an ob.gyn. with, a Dublin-based consortium of eight Irish fetal medicine centers that is examining ways to improve delivery outcomes.
The study used “a pure population of pregnancies with unsuspected fetal macrosomia,” he explained.
Dr. Galvin and his colleagues used data collected in, a prospective study run by the Perinatal Ireland multicenter consortium with the primary goal of determining whether late-pregnancy fetal head circumference can predict labor dystocia and intrapartum cesarean delivery. They examined two secondary outcomes: the reliability of ultrasound to estimate fetal size, and the consequences of fetal macrosomia when it is not recognized until delivery is already underway.
The study enrolled 2,336 nulliparous women with singleton pregnancies that ranged from the start of 39 weeks’ gestational age through the end of 40 weeks. The women underwent a standard ultrasound examination to assess fetal biometrics. The study excluded pregnancies with an estimated fetal size greater than 5,000 g. Mothers carrying a fetus estimated to be less than 4,000 g constituted 88% of the study group, with 12% carrying pregnancies with an estimated fetal weight greater than 4,000 g.
The ultrasound examination worked reasonably well for ruling out macrosomia, with an 89% rate of correctly identifying fetuses with a birth weight of less than 4,000 g. Near-term ultrasound was less useful for a positive identification of macrosomia; it flagged 58% of the fetuses born heavier than 4,000 g.
Analysis of delivery mode showed that infants born weighing more than 4,000 g had a statistically significant 56% reduced rate of spontaneous vaginal deliveries compared with smaller neonates, a 63% greater rate of cesarean deliveries, and a 49% greater rate of operative vaginal deliveries, compared with small babies, Dr. Galvin. All three between-group differences were statistically significant.
The analysis also showed that compared with the smaller babies, the larger neonates were twice as likely to be born during prolonged labor of more than 12 hours. Delivery of larger neonates was also twice as likely to trigger postpartum hemorrhage. But deliveries of larger babies had no significant link with increased rates of neonatal intensive care admissions, anal sphincter injuries, shoulder dystocias or birth injuries, compared with deliveries of smaller babies.
Dr. Galvin reported having no financial disclosures.