Obstetric and neonatal outcomes of spontaneous labor with a large fetus depending on the gestational age

Background: Fetuses that are large for gestational age pose a risk of macrosomia, particularly when excessive growth occurs at full-term gestational age. However, not all large-for-gestational-age fetuses are born with macrosomia, which can result from either earlier delivery or a decrease in growth rate over time. Existing studies on the outcomes of labor induction for fetal macrosomia typically focus on fetal body weight at the time of induction, with few considering gestational age. Investigating the outcomes of spontaneous labor in large-for-gestational-age fetuses can help to identify the most favorable gestational age for delivery and reduce the incidence of such cases.

Objective: To study obstetric and neonatal outcomes of spontaneous labor with a large fetus depending on gestational age.

Materials and methods: This retrospective cohort study included 571 healthy primiparous women who were divided into two groups based on ultrasound findings. The control group comprised pregnancies with fetal sizes ranging from the 10th to the 80th percentile (n=345), while the study group included pregnancies with large-for-gestational-age fetuses above the 90th percentile (n=226). Each group was further divided into four subgroups: subgroup 1 – gestational age 37 weeks, subgroup 2 – 38 weeks, subgroup 3 – 39 weeks, and subgroup 4 – 40 weeks.

Results: The overall rates of operative deliveries and cesarean sections were significantly higher in the study group – 23% versus 11.6% for operative deliveries (p=0.0004) and 21.2% versus 8.7% for cesarean sections (p<0.0001). Among women with large fetuses at 36–37 weeks of pregnancy, every third observation at 38–39 weeks resulted in the birth of a child with macrosomia, whereas this occurred every in second observation at 40 weeks. The lowest cesarean section rates and occurrence of large fetuses were noted at 38–39 weeks.

Conclusion: Pregnant women with an estimated fetal weight at the 90th percentile or higher, determined by ultrasound at 36–37 weeks, are at an increased risk of higher cesarean section rates and the birth of large fetuses. The optimal delivery window for these cases was between the end of the 37th week and the beginning of the 39^th week, during which the lowest cesarean section rates were observed.