Second-versus first-born twin: comparison of short- and long-term outcomes

An ancient description of the competition between twins for first breath is found in the biblical story of Jacob and Esau (Genesis 25:26) when Jacob pulled his older brother's heel in the hope of becoming firstborn but to no avail. In this study, we sought to evaluate the short and long-term outcomes of twin pairs, comparing between the second- and first-born twin.

A population-based cohort study, including dichorionic twin deliveries occurring between the years 1991 and 2021 at Soroka University Medical Center. A General estimation equation (GEE) was applied to adjust for confounders. The incidence of offspring's hospitalizations due to various medical conditions was compared. Kaplan-Meier survival analyses compared cumulative morbidity. Cox proportional hazards models were used to control for confounders.

5507 twin deliveries met the inclusion criteria. Second-born twins had higher rates of cesarean deliveries, statistically significant in the GEE multivariable analysis. More first-twin fetuses were experiencing non-reassuring fetal heart rate patterns, although other obstetrical outcomes as well as mortality rates were comparable between groups. Second twins weighed lower than their older sibling (mean difference 33 g) and were more frequently SGA and low birthweight (1500-2500 g); (p < 0.05). Later during childhood, offspring of twin deliveries experienced notable morbidity due to infectious (23.8-24.1%), respiratory (10.5-10.9%), neurological (7.0-7.8%) and cardiovascular pathologies (1.7-1.9%) during childhood, that was unaffected by birth order.

Other than birthweight differences, the birth order of dichorionic twins is not associated with adverse neonatal health indices, nor does it predict excess risk for morbidity during childhood.