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% of all new-born babies are male in the western world, the proportion of female offspring rises dramatically among smokers.

The study of 9,000 pregnancies in Liverpool found a startling imbalance in the number of girl babies among parents who smoke. Mothers who smoked in pregnancy were one-third less likely to have male children than those who did not. 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.

"This is a new and very interesting finding. Smoking accentuates the gender ratio, " said Alison Poulton, senior lecturer in paediatrics at the University of Sydney, Australia. "The next stage is to look at the reasons for these ratios and to conduct further research in other parts of the world."

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 cigarette smoke from partners are less likely to have boys.

The highest number of male babies were found in smoke-free households in which there was no smoke exposure during the pregnancy.

The research, by a team of paediatricians led by Professor Bernard Brabin and Dr Ali Delpisheh of the child and reproductive health group at the Liverpool School of Tropical Medicine, concluded that "maternal smoking exposure during pregnancy either preferentially increases early male foetal deaths or reduces the frequency of conceiving male children".

The researchers believe that substances contained in cigarettes, such as nicotine, inhibit sperm carrying a male chromosome from fertilising eggs and implanting.

"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 Brabin. "We don't know what precisely causes it. The hypothesis is that sperm cells carrying the Y chromosome are more sensitive to unfavourable smoking related changes in the mother. Smoking reduces oestrogen and causes changes to the mothers' cervix.

It could affect the implantation of the male fertilised egg into the uterine wall.

The message is clear: if you want an increased chance of a male baby don't smoke during pregnancy."