Perinatal outcome and long-term hospitalization of triplets according to birth order

The association between birth order and adverse perinatal outcomes has been well studied in twin pregnancies. However, little is known about the differences in immediate perinatal outcomes as well as long-term hospitalization of the offspring in triplet pregnancies according to their birth order. As such, we aimed to assess the differences in immediate perinatal outcomes and long-term hospitalizations among triplets by their birth order.

In a retrospective hospital-based cohort study, immediate perinatal outcomes and long-term hospitalizations were compared among triplet siblings according to their birth order. Deliveries occurred between the years 1991 and 2021 in a tertiary medical center. The study groups were followed until 18 years of age for cardiovascular, respiratory, neurological, and infection-related hospitalizations. Generalized estimation equation (GEE) models were used to control for confounders. Kaplan-Meier survival curves were used to compare cumulative long-term hospitalization incidences and Cox proportional hazards models were performed to control for confounders.

The study included 117 triplet deliveries. Rates of small for gestational age (SGA) infants increased linearly by birth order (6.0%, 7.7%, and 15.4% for the first, second, and third siblings, respectively; p-value for trends = 0.035). Using a GEE model controlling for maternal age, being born third in a triplet pregnancy was independently associated with SGA (third vs. first sibling, adjusted OR 3.0, 95% CI 1.38-6.59, p = 0.005). No significant differences in cardiovascular, respiratory, neurological, and infection-related hospitalizations were noted among the first, second, and third siblings. Likewise, using Kaplan-Meier survival analyses, no significant differences in the cumulative incidence of long-term pediatric hospitalizations were noted between the siblings. In Cox proportional hazards models, controlling for weight and gender, birth order in a triplet pregnancy did not exhibit an association with long-term hospitalizations of the offspring.

Despite the association between birth order and SGA, birth order in triplets does not seem to have an impact on the risk for long-term offspring hospitalization.