Smoking boosts risk of tubal pregnancy.

Women smokers who become pregnant run a 40 percent greater risk of tubal pregnancy than their nonsmoking counterparts, reports a team led by Andy Stergachis of the University of Washington in Seattle.

The researchers, who describe their work in the Feb. 15 American Journal of Epidemiology, compared the smoking histories of 274 women who had tubal pregnancies and 727 randomly selected women of reproductive age, some of whom were pregnant. Tubal pregnancies occur when a fertilized egg becomes implanted in the wall of one of the fallopian tubes instead of descending to the uterus.

The investigators postulate that the nicotine in cigarettes may interfere with the mechanisms that normally transport a fertilized egg to the womb. They cite previous studies in animals and humans showing that nicotine impairs tiny hairlike structures in the fallopian tubes that normally sweep eggs into the uterus.

Interestingly, the researchers found that heavy smokers had a smaller chance of a having a tubal pregnancy than light smokers. They surmise that this may reflect the lower pregnancy rates of smokers, particularly heavy smokers.

In four previous studies, other researchers had found that the risk of tubal pregnancy was 50 to 130 percent greater in smokers than in nonsmokers. The new work confirms this hazardous link with a more stringently controlled study, says Delia Scholes of the Group Health Cooperative of Puget Sound in Seattle, who helped conduct the recent research.