OBJECTIVE: To determine the effects of fetal sex and mode of delivery on the risk of developing RDS.

STUDY DESIGN: This was a hospital-based cohort study of consecutive live births from 1978 to 1996. Maternal and neonatal variables, including fetal sex, route of delivery and standardized diagnosis of RDS by a senior neonatologist, were analysed using the computerised Obstetrical and Neonatal Database. Multiple logistic regression was used to assess the independent role of sex and mode of delivery in the development of RDS and to control for gestational age, antenatal exposure to corticosteroids, maternal diabetes, intrauterine growth restriction, chronic and pregnancy-induced hypertension, prolonged rupture of membranes, abnormal presentation, low one-minute Apgar score, multiple gestation and order of delivery (for multiples).

RESULTS: There were 65,389 live births (including antenatal references) of known gestational age, 924 (1.4%) of whom developed RDS. The incidence of RDS was 1.7% for males and 1.1% for females. After controlling for all other perinatal variables, the odds ratio (OR) was 1.9 (1.6-2.3) for males compared to females. There was no sex difference prior to 28 weeks, with steadily increasing male relative risk (RR) with advancing gestational age to reach a peak of 3.3 RR at 37-39 weeks. RDS developed in 0.74% delivered vaginally, 3.1% delivered by cesarean section (C/S) in labor and 5.0% delivered by C/S with no labor. After controlling for all other perinatal variables, the OR of C/S in labor was 2.3 (1.8-3.0) and of C/S with no labor 3.8 (3.0-4.8) compared with vaginal deliveries. The RR associated with C/S was present at all gestational ages but became greater (6.7) at 37-39 weeks. There were no cases of RDS at or after 40 weeks.

CONCLUSIONS: Fetal sex influences the development of RDS and its role is dependent on gestational age, becoming more important near and at term. The route of delivery and presence of labor also influence the development of RDS, with a greater influence at term. These findings should be considered when planning deliveries before 40 weeks.