Background: Neonatal mortality and morbidity are sex biased in low birth weight infants. The "Y chromosome effect" has been suggested to be responsible for these maturational differences.
Objective: To examine the association of sex and neonatal outcomes.
Design and methods: A retrospective observational study. Data on all low birth weight infants who survived for >48 hrs were analyzed. Neonatal outcomes were compared between male and female infants. A regression model was used to detect the influence of sex on outcomes after controlling for confounders. Analysis was repeated after stratification of infants into three groups: group A (<1000 g), group B (1000-1499 g), and group C (1500-2499 g).
Results: A total of 833 infants were included in this study; 419 female infants and 414 male infants. Male infants had an increased rate of overall intraventricular hemorrhage (IVH) (12.2% vs. 7.2%, p = .02) and IVH grades 3-4 (4.8% vs. 2.3%, p = .04). In addition, male infants had higher bilirubin levels (10.19 +/- 3.1 mg/dL vs. 9.32 +/- 2.94 mg/dL, p = .001). In a regression model, male sex continued to have significant influence on IVH, IVH grades 3-4, death, and bilirubin. In group A, male infants had a significantly increased prevalence of death (regression coefficient, 1.82 +/- 0.65; p = .005) that could not be explained by the increased prevalence of IVH (p = .18) in regression analysis. In group B, male sex was significantly associated with a higher bilirubin level (regression coefficient, 0.94 + 0.3; p = .002). In bivariate analyses, IVH and IVH grades 3-4 were significantly higher in male compared with female infants (19.8% vs. 3.9%, p < .0001) and (8.5% vs. 0.97%, p = .02), respectively, but these differences lost significance in multiple-regression analysis. In group C, male sex positively influenced the prevalence of IVH (regression coefficient, 1.7 +/- 0.57; p = .003). Bilirubin measured higher in male infants (11.38 +/- 2.87 mg/dL vs. 10.19 +/- 3.22 mg/dL, p = .0004), but the difference lost significance in regression analysis (regression coefficient, 0.21 +/- 0.31; p = .5).
Conclusions: Bilirubin, IVH, and death were significantly higher in male infants. In subgroup analysis, significance was retained in group A (<1000 g). Whether a single biological factor is responsible for these differences or perhaps a multi-causal process involving a complex interaction of physiologic, environmental, and pathologic responses needs to be further addressed in future research.