Perinatal mortality in second- vs firstborn twins: a matter of birth size or birth order?

Second-born twins on average weigh less than first-born twins and have been reported at an elevated risk of perinatal mortality. Whether the risk differences depend on their relative birth size is unknown. The present study aimed to evaluate the association of birth order with perinatal mortality by birth order-specific weight difference in twin pregnancies.

In a retrospective cohort study of 258,800 twin pregnancies without reported congenital anomalies using the US matched multiple birth data 1995-2000 (the available largest multiple birth dataset), conditional logistic regression was applied to estimate the odds ratio (OR) of perinatal death adjusted for fetus-specific characteristics (sex, presentation, and birthweight for gestational age).

Comparing second vs first twins, the risks of perinatal death were similar if they had similar birthweights (within 5%) and were increasingly higher if second twins weighed progressively less (adjusted ORs were 1.37, 1.90, and 3.94 if weighed 5.0-14.9%, 15.0-24.9%, and ≥25.0% less, respectively), and progressively lower if they weighed increasingly more (adjusted ORs were 0.67, 0.63, and 0.36 if weighed 5.0-14.9%, 15.0-24.9%, and ≥25.0% more, respectively) (all P < .001). The perinatal mortality rates were not significantly different in cesarean deliveries or preterm (<37 weeks) vaginal deliveries but were significantly higher in second twins in term vaginal deliveries (3.1 vs 1.8 per 1000; adjusted OR, 2.15; P < .001).

Perinatal mortality risk differences in second vs first twins depend on their relative birth size. Vaginal delivery at term is associated with a substantially greater risk of perinatal mortality in second twins.