This retrospective cohort study evaluated pregnancy outcomes and similarities between pairs of nulliparous sisters with a singleton fetus who delivered between 2013 and 2020. The “Sister-1 group” was defined as the sibling who delivered first, while the “Sister-2 group” included the siblings who gave birth after Sister-1. Obstetrical complications and delivery outcomes were compared. The relative risk for recurrence of a complication in Sister-2 was calculated. The study included 743 sister pairs. There were no between-group differences in maternal BMI, gestational age at delivery, gravidity, smoking, or epidural rates. The Sister-2 group was older than the Sister-1 group (26.4 ± 5 vs. 25.8 ± 4.7 years, respectively, p = 0.05). Higher birthweights and more large-for-gestational-age infants characterized the Sister-2 group compared with the Sister-1 group (3241 ± 485 g vs. 3148 ± 536 g, p < 0.001 and 7.7% vs. 4.8%, p = 0.025, respectively). There were no between-group differences in the rate of small-for-gestational-age, gestational diabetes, hypertensive disorders, pre-term births, vacuum extraction, or cesarean deliveries. Logistic regression analysis found that if Sister-1 underwent vacuum extraction, her sibling had an increased risk for vacuum delivery (adjusted RR 3.03, 95% CI 1.4−6.7; p = 0.003) compared with those whose sibling (Sister-1) did not. There was a three-fold risk of vacuum extraction delivery between sisters. This finding could be related to biological inheritance, environmental factors, and/or psychological issues that may affect similarities between siblings’ delivery outcomes.
Keywords: dysfunctional labor; familial predisposition; nullipara; operative delivery; sisters.