Background: To describe the association between maternal hypertension (chronic and gestational, MH) and mortality in very preterm singletons and twins, focusing on how estimates depend on gestational age (GA) and size at birth.
Methods: We estimated relative risks of in-hospital death in 12,320 singletons (MH: 22.4%) and 4381 twins (MH: 10.6%) born at 23-29 weeks in the Italian Neonatal Network (89 hospitals, 2008-2016).
Results: Babies with MH had higher GA and were more frequently small-for-gestational age (SGA), especially singletons. In crude analyses, MH was associated with lower mortality in singletons. In multivariable analyses, the effects of GA and size differed between twins and singletons with and without MH. The best-fitting models included continuous birth weight (rather than SGA) and were stratified by GA. In these models, MH was associated with lower mortality in singletons-but not twins-born after week 25.
Conclusions: In this cohort of very preterm infants, the association between MH and mortality differed between singletons and twins and across strata of GA at birth. These estimates cannot be interpreted causally, but suggest that, from a descriptive/predictive standpoint, singletons with MH born after week 25 have lower mortality than singletons born to women without MH.