Maternal Smoking and Sex Distribution Among Live Births in Western Sydney: A Population-Based Cohort Study

Maternal smoking may affect fetal survival, but evidence regarding offspring sex is inconsistent. We estimated the male proportion among live births and did not assess sex determination or pregnancy loss before the perinatal reporting threshold.

To examine whether smoking during the first half of pregnancy was associated with male live birth and whether the association differed by smoking intensity.

We analysed singleton live births of Western Sydney residents in the New South Wales Perinatal Data Collection, 2016-2023. Smoking was classified as non-smoking, ≤ 10 cigarettes/day, or > 10 cigarettes/day. Logistic regression estimated odds ratios (ORs), adjusted for calendar year and maternal demographic, socioeconomic and clinical characteristics.

Of 119 756 eligible births, 721 (0.6%) had unknown smoking information, leaving 119,035 births. Male births comprised 51.6% among non-smokers, 52.2% among women smoking ≤ 10 cigarettes/day, and 52.4% among women smoking > 10 cigarettes/day. Crude differences were + 0.60 and + 0.78 percentage points (p = 0.608). The adjusted model included 117,745 births; adjusted ORs were 1.02 (95% CI 0.96-1.08) and 1.02 (95% CI 0.90-1.16), respectively. The heavier-smoking estimate was imprecise (n = 964).

Lower-intensity smoking was not meaningfully associated with male live birth. Estimates for > 10 cigarettes/day were imprecise, and moderate effects in either direction could not be excluded. Findings apply only to live births reaching the reporting threshold.