Trial of labor when twin B is slightly larger than twin A: A Danish prospective cohort study

To assess whether a trial of labor at term in twin pregnancies, where twin B is at least 250 g heavier than twin A, is associated with increased risk of emergency cesarean section or mixed-mode birth (vaginal delivery of twin A followed by cesarean of twin B). Existing guidelines on this topic are based on heterogeneous cohorts that combine planned cesarean, emergency cesarean, and mixed-mode births, limiting direct conclusions on trial of labor outcomes.

In this prospective cohort study, we included twin pregnancies referred to a Danish tertiary center from 2007 to 2019. Only those with gestational age ≥ 37 + 0 weeks and birth weight discordance <25% were eligible. Among 302 women undergoing trial of labor, 59 had a weight discordance ≥250 g with twin B as the larger fetus ("twin B larger"), while 243 formed the control group ("twins equal" or "twin A larger"). Primary outcomes were emergency cesarean section and mixed-mode birth. Secondary outcomes included blood loss ≥1000 mL, Apgar score <7 at 5 min, umbilical artery pH <7.1, and neonatal length of stay.

The rate of emergency cesarean section was higher in the "twin B larger" group (29%) compared with controls (19%, P = 0.09). Mixed-mode birth occurred in 8.5% of the "twin B larger" group and 11.5% of controls (risk difference, -3.0% [95% CI, -11.2% to 5.1%]). No significant differences were observed in secondary neonatal outcomes between groups.

In twin pregnancies at term where twin B is slightly heavier than twin A, a trial of labor is not associated with a clinically meaningful increase in the risk of mixed-mode birth. While the emergency cesarean section rate was numerically higher, this did not reach statistical significance and should not in itself contraindicate vaginal delivery. Further research with larger sample sizes is warranted to confirm these findings.