Morbidity and Mortality of Very-Low-Birthweight Twin Infants according to Their Sex and the Sex of the Co-Twin: A Retrospective Cohort Study.

INTRODUCTION: The concept of male disadvantage regarding the prognosis of premature newborns was introduced more than half a century ago, and it has been corroborated over time. However, the influence of the sex of one twin on the outcomes of the other has yielded contradictory results. OBJECTIVE: The aim of the study was to determine if, in twin pregnancies of VLBW infants, the outcomes of one twin are modified by the sex of the co-twin. METHODS: A multicentre retrospective study of a cohort of infants admitted to the collaborating units of the Spanish SEN1500 neonatal network was conducted. Liveborn VLBW twin infants, from 23+0 to 31+6 weeks of gestational age (GA), admitted from 2011 to 2020 were included. Outborn patients, infants with major congenital anomalies, and cases with only one twin admitted were excluded. The main outcomes were survival until first hospital discharge, survival without moderate or severe bronchopulmonary dysplasia (BPD), survival without major brain damage (MBD), and survival without major morbidity. Incidence rate ratios (IRR) and 95% confidence intervals (CI) were calculated. RESULTS: 2,111 twin pairs were included. Male infants exhibited worse outcomes than females (IRR; 95% CI) regarding survival (0.96; 0.94, 0.98), survival without moderate or severe BPD (0.89; 0.86, 0.93), survival without MBD (0.94; 0.91, 0.97), and survival without major morbidity (0.87; 0.81, 0.93). Differences disappeared when the co-twin was a female infant: survival (1.00; 0.97, 1.03), survival without moderate or severe BPD (0.96; 0.91, 1.01), survival without MBD (0.99; 0.95, 1.04), and survival without major morbidity (0.94; 0.85, 1.03). Results for female infants did not change significantly with co-twin sex. CONCLUSIONS: Among VLBW twins from 23+0 to 31+6 weeks of GA, male infants have higher risk of morbidity and mortality overall. In cases of pregnancies with different-sex foetuses, males seem to improve their results, while these do not change for females. The underlying mechanism of this influence deserves further investigation.

Survival and Survival without Major Morbidity Seem to Be Consistently Better throughout Gestational Age in 24- to 30-Week Gestational Age Very-Low-Birth-Weight Female Infants Compared to Males.

INTRODUCTION: Several studies showed advantages in outcomes for very-low-birth-weight (VLBW) female infants. It has been suggested that recent advances in perinatal care might have benefited boys relatively more than girls, making differences disappear. OBJECTIVES: The aims of the study were (1) to determine if sex differences in survival and survival without morbidity in VLBW infants are still present in the context of more advanced perinatal care and (2) to know whether these differences are consistent throughout gestational age (GA). METHODS: Retrospective cohort study in seven countries participating in the Spanish SEN1500 and the South American NEOCOSUR neonatal networks. We included VLBW infants 24-30 weeks' GA, born alive without major congenital anomalies (2013-2016). Major morbidity, survival, and survival without morbidity were compared between male and female infants overall and stratified by GA. RESULTS: 10,565 patients were included: 5,620 (53.2%) males and 4,945 (46.8%) females. Female infants exhibited a lower incidence rate ratio (95% CI) of respiratory distress syndrome: 0.91 (0.88, 0.94), necrotizing enterocolitis: 0.83 (0.74, 0.93), major brain damage: 0.79 (0.72, 0.86), moderate-severe bronchopulmonary dysplasia (BPD): 0.77 (0.72, 0.83), higher survival: 1.03 (1.01, 1.05), survival without BPD: 1.11 (1.07, 1.16), survival without major brain damage: 1.05 (1.02, 1.08), and survival without major morbidity: 1.14 (1.07, 1.21). Survival and survival without morbidity were almost consistently favourable to females throughout GA. CONCLUSIONS: Our findings suggest that perinatal results continue to be favourable for VLBW female infants in the context of current perinatology, and that they are almost consistent throughout GA.