A prospective study of fertility was conducted from 1977 to 1982. Analysis of the relation between cigarette smoking and occurrence of a pregnancy was performed on 1887 couples. The actuarial pregnancy rate was 82.9% at the end of a year. Cigarette smoking by both members of the couple was found to be related to decreased fertility when this factor was considered alone. But after including all the confounding covariates in a Cox semi-proportional hazards model for survival data, no relationship remained between cigarette smoking and fertility. Odds ratios were respectively 0.86 (95% confidence interval: 0.63, 1.19) and 0.99 (0.85, 1.14) for women smoking and men smoking. This could suggest that the relationship found by other authors might be at least partly explained by a relation between cigarette smoking and other factors related to fertility.
PIP: The association between cigarette smoking and fertility was investigated in 1887 couples recruited through newspaper and radio announcements. Criteria for entry into the study included: 1) absence or interruption of contraception during the study; 2) the existence of at least 1 eligible cycle; 3) interpretable cycles; 4) cigarette smoking; and 5) no obvious signs of infertility. 1164 (62%) of these couples achieved a pregnancy during the 5-year study period. The actuarial pregnancy rate was 82.9% at the end of the 1st year--higher than the crude rate due to drop-outs. At the end of 1 year, 85% of nonsmoking controls had conceived compared with 70% of the smoking women. It was not possible to assess the dose-response relationship since only 20 subjects smoked more than 20 cigarettes/day; the vast majority smoked under 10/day. When confounding factors (couples who had begun to attempt to conceive before entering the study, women whose last form of contraception had been the pill or IUD, and women whose husbands were smokers) were included in the Cox model, there was no longer any significant difference in fertility between smokers and nonsmokers. The odds ratios were 0.86 (95% confidence interval, 0.63-1.19) for women smoking and 0.99 (0.85-1.14) for men smoking. It is suggested that the effect of tobacco on fertility found in previous studies may be at least partially due to behavioral factors related to cigarette consumption, even if it is not possible to totally eliminate the possibility that the apparent association between fertility and confounding factors could be explained by an association between these factors and cigarette smoking.