Introduction Over the past few decades, birth registries of developed countries have shown a significant decline in the male to female (M: F) sex ratio. The reason for this reduction is unknown, although increasing exposure to cigarette smoke at home has been reported as a possible contributor.

Aim and Methods This was a retrospective cohort analysis to investigate the association of parental smoking with the offspring ratio among 8960 women who delivered singleton live births at the Liverpool Women’s Hospital between 1998–2003 and among mothers of 3038 children who were part of a community respiratory health survey conducted in 1998 and 2006.

Results The prevalence of maternal pregnancy smoking was 32.7% and 32.4% for the hospital and community sample, respectively. The male to female ratio was 1.08 for the hospital sample and 0.97 for the community sample. The chance of female birth with pregnancy smoking was 1.2 times increased with the hospital sample and 1.4 times increased with the community sample. There was a significant dose response association of maternal smoking with the fetal sex ratio, with heavy smokers (>10 cigarettes/day) more likely to deliver a female baby than light smokers for both community and hospital samples (p = 0.001 and p = 0.05, respectively). There was an independent association of parental smoking with a female birth, after controlling for year of birth registration, household deprivation, alcohol exposure and body mass index (adjusted odds ratio:1.41,1.12 to 1.92, p = 0.001) with the hospital sample.

Conclusion Maternal cigarette smoke exposure prior to and during pregnancy increased the likelihood of a female birth.