Fetal biparietal diameter as a potential risk factor for prolonged second stage of labor: A retrospective observational cohort study.

Prolonged second stage of labor is a common abnormality of labor progression. Very little research exists regarding the relationship between prolonged second stage of labor and antepartum sonographic fetal head biometry parameters, especially fetal biparietal diameter (BPD). Fetal BPD assessment is essential for estimating fetal weight, and these measurements are readily available to Japanese obstetricians. We conducted a retrospective observational cohort study to evaluate the association between BPD fetal Z-score and prolonged second stage of labor in a Japanese cohort. Individual BPD data measured using a GE Voluson 730 expert ultrasound system (GE, Healthcare Japan, Tokyo, Japan) were converted to Z-scores for a particular gestational age. After excluding patients with multiple pregnancies and emergency or elective cesarean sections, a total of 2,711 (nulliparity, n = 1341) Japanese women who delivered at term were included. We analyzed the incidence of prolonged second stage of labor and the association between BPD Z-score measured <7 days before delivery and prolonged second stage of labor by parity. The overall incidence of prolonged second stage of labor was 18.3% (246/1,341) in nulliparous women and 4.6% (63/1,370) in multiparous women. In nulliparous women, multivariable analysis indicated that BPD Z-score was significantly associated with prolonged second stage of labor (adjusted odds ratio, 1.18; 95% confidence interval, 1.02-1.37). Kaplan-Meier survival analysis showed that at each time point during the second stage of labor, the percentage of women who had not yet delivered was higher among those who delivered neonates with large BPD Z-scores than among those who delivered neonates with smaller BPD Z-scores. On the contrary, in multiparous women, BPD Z-score was not statistically associated with prolonged second stage of labor. Our results suggest that considering BPD Z-score is helpful in the management of nulliparous women who are at risk of developing a prolonged second stage of labor.