Smoking Cuts Pregnancies in Half

Even a Small Number of Cigarettes Affects Fertility, Study of Mothers Indicates

by Patricia McBroom

Even in very low doses, cigarette smoking substantially interferes with the ability of women to become pregnant. From one to nine cigarettes per day are enough to inhibit conception, according to a new study from the School of Public Health.

Smoking less than half a pack a day led to approximately a 50 percent reduction in fertility at the end of one year, researchers found.

The impact was just as great among light smokers as among heavy smokers in this large sample of first-time mothers.

Data on 1,341 women who first gave birth between 1959 and 1966 were analyzed by Ethel Alderete, a doctoral student; Brenda Eskenazi, professor of public health; and Robert Sholtz, a statistician. They used data from personal interviews collected in the early 1960s from first-time mothers that includes information on how long it took women to become pregnant. The original study, carried out by the School of Public Health and Kaiser Permanente, measures the time lag between last contraceptive use and first missed menstrual period.

Selecting only those women who consciously planned to become pregnant, Alderete and Eskenazi discovered that smokers took about twice as long as non-smokers to become pregnant.

"Smoking reduces your chances of becoming pregnant within 12 months by 50 percent, and the effect is there with only a few cigarettes a day," said Eskenazi. She added that more research should be done to confirm this unusual lack of a dose-effect relationship.

The study, published in the July issue of the journal Epidemiology, is particularly notable in that it separates out the effects of coffee drinking and smoking, which have until now been confused in studies of fertility.

Many smokers also drink coffee, and while lowered fertility rates have been reported before, scientists could not be sure that cigarettes were the culprit.

This study demonstrated that coffee use alone had no effect on fertility.

The deleterious effects were all due to cigarettes, said Alderete.

It is not known whether women recover full fertility if they stop smoking.

"Maybe there is good recovery, but we don't know that." said Eskenazi. She said the biological mechanism through which smoking impairs fertility is unknown. Cigarettes could be acting anywhere along the reproductive path, from hormonal effects in the hypothalamus, to reduced ovulation, reduced motility in the tubes or impaired implantation of the fetus.

"If you are 35 years old when you first try to conceive, this additional insult to the fertility system may be enough to make you infertile," said Eskenazi.

In this study, most of the women were younger than 25, with healthy reproductive systems. Those with reproductive problems or who had taken more than two years to conceive were removed from the analysis. About 41 percent of the total group smoked cigarettes, a common habit in the early '60s.