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Background: A recent decline in the male:female (M:F) sex ratio may relate to pregnancy cigarette smoke exposure., Aim: To assess trends and cigarette exposure dose-response effects on the sex ratio., Subjects and Methods: A retrospective analysis was carried out of deliveries at the Liverpool Women's Hospital between 1998 and 2003, and of deliveries reported in community surveys from the same area in 1998 and 2006., Results: For the hospital sample, the M:F sex ratio was 1.14 if no parent smoked, and 0.77 when both parents smoked during the mother's pregnancy (p < 0.001). Heavy maternal smokers (>10 cigarettes per day) were more likely to deliver a female baby than light smokers (p < 0.001). Smoking was associated with increased likelihood of female birth controlling for birth year, socio-economic status, alcohol exposure, maternal haemoglobin and body mass index (adjusted OR: 1.41, 95% CI 1.12-1.92, p < 0.001). In the community sample controlling for socio-economic status the ratios were 1.13 (95% CI 1.03-1.24, p = 0.015) in 1998 and 1.31 (95% CI 1.16-1.48, p < 0.001) in 2006. Secular trends showed decreasing ratios in hospital and community samples for both smokers and non-smokers., Conclusion: Pregnancy cigarette smoking increased the proportion of female births with evidence for a dose-response association.