People who smoke are more likely to have a female child; new research has revealed that the chance of having a male baby drops by almost half if both parents smoke during early pregnancy. The study was carried out by pediatricians at the Liverpool School of Tropical Medicine, and involved 9000 women who delivered children in 1998 and 2003. The results showed that women who smoked were one third less likely to have a male baby; women who were exposed to secondhand smoke also had fewer boys than girls.

Doctors believe that the reason lies in the effect nicotine and other substances in cigarettes have on the reproductive organs. Nicotine may damage sperms carrying the Y chromosome which are responsible for producing male babies. Changes in the mother’s hormones and cervix also make it more difficult for male embryos to implant in the uterus. Those male embryos that do implant are more likely to be killed by the toxic effects of nicotine in the mother’s blood.

The results have caused concern in the scientific community; millions of people smoke worldwide and the effect on population demographics can be serious.