Parents who are smokers when they conceive a child are far more likely to have girls than boys, an extensive study by paediatricians has found. The chance of having a male baby drops by almost half if both parents smoke during early pregnancy, amid fresh evidence that smoking could “kill” male foetuses in the womb.

Smoking not only reduces the chances of conceiving a male child, but could stop male embryos implanting in the womb and cause miscarriages.

While almost 52 per cent of all new-born babies are male in the western world, the proportion of female offspring rises dramatically among smokers. The study into 9,000 pregnancies in Liverpool found a startling imbalance in the number of girl babies among parents who smoke. Mothers who smoked during pregnancy were one-third less likely to have male children than mothers who did not smoke. If the father also smoked, and if factors such as the health and age of the mother were accounted for, the chances of having a male child reduces by almost half.

The research by paediatricians at the Liverpool School of Tropical Medicine has caused excitement in the international scientific community, which says it raises serious questions about the effect of smoking on population balance.

“There are a lot of studies looking at birth weight in relation to smoking but this shows that mothers who smoke are more likely to have a female child,” said Alison Poulton, senior lecturer in paediatrics at the University of Sydney, Australia.

The study looked at 9,000 women who delivered children at Liverpool Women’s Hospital between 1998 and 2003. It found heavy smokers were the most likely to deliver female babies, but that passive smoking may also affect the gender of a child. Mothers who do not smoke but are exposed to cigarettes from partners are less likely to have boys.

The research, by a team of paediatricians led by Professor Bernard Brabin, found that substances contained in cigarettes, such as nicotine, inhibit sperm carrying male chromosomes from fertilising eggs.

“The study aimed to assess whether maternal smoking during pregnancy affects the likelihood of the offspring being male or female. When we adjusted for factors statistically like maternal age and if both parents smoked they had almost double the chance of having a female baby,” said Mr Brabin, professor of tropical paediatrics. “We don’t know what causes it. The hypothesis is that sperm cells carrying the Y- chromosome which are responsible for male children are more sensitive to unfavourable smoking related changes in the mother. Smoking reduces oestrogen and causes changes to the mother’s cervix. The message is clear: if you want an increased chance of a male baby, don’t smoke during pregnancy.”