Lean and, to a much greater extent, obese women who smoke require a longer time to conceive than do their normal-weight counterparts, according to the results of a population-based survey conducted in five European countries.1 The odds that women who wanted to become pregnant would take at least 9.5 months to conceive were nearly doubled for smokers who, on the basis of their body mass index, were classified as lean and rose by a factor of almost 12 for those who were obese. No associations between body mass and delayed conception were found among nonsmokers. The analysis is one of the few to have examined the effects of excess weight on delayed conception, and it is the first to draw on a sample from the general population rather than from select groups.
The data come from a 1992 survey of pregnant women in Denmark, France, Germany, Italy and Sweden. Respondents provided information on their background characteristics; diseases and conditions that may affect fertility; height and weight; reproductive history; frequency of intercourse; contraceptive use; smoking, caffeine intake and alcohol consumption; and the length of time it took them to become pregnant. Using the women's reports of their height and weight, the researchers calculated their body mass index and classified them as either lean (if the index value was below 20 kg/m2), normal-weight (20-24.9 kg/m2), overweight (25-29.9 kg/m2) or obese (30 kg/m2 or greater). Only women whose pregnancies were planned were included in the analyses, yielding a sample of 2,587.
Some 16-18% of normal-weight and overweight women said that it had taken them at least 9.5 months to conceive, compared with 22% of lean and 31% of obese women. The average time it took to become pregnant was significantly longer among obese women (11 months) than among others (7-8 months). Initial analyses revealed several differences among women according to their weight classification at the time they began trying to conceive. Overweight and obese women had had less education than others, had been less likely have a job and had been pregnant more times; they also reported the lowest alcohol consumption. Obese women had been the youngest and had smoked the most cigarettes.
In multivariate analyses that took into account the effects of potentially confounding factors, the only significant interaction was between body mass index and smoking status; therefore, the researchers examined results separately for smokers and nonsmokers. They found that among smokers, obese women were 11.5 times as likely as those with a normal weight to have spent at least 9.5 months trying to conceive, and lean women were 1.7 times as likely as normal-weight women to experience delayed conception; no effect emerged for overweight smokers. Among nonsmokers, the odds of delayed conception were not associated with body mass.
Additional analyses generally confirmed these findings. When the investigators redefined delayed conception to refer to waiting times of 12.5 and 15.5 months, obese smokers still had substantially elevated odds of this outcome; the results for lean women, however, were not statistically significant. Furthermore, the same patterns were seen when the calculations included both planned and unplanned pregnancies, and when they were restricted to women who were pregnant for the first time.
The investigators comment that the interaction between smoking and body mass index may have important implications "for preventive counseling of women who intend to become pregnant." However, they observe that "for some women, weight reduction may be more difficult in the short term than smoking cessation." When this is the case, they suggest that interventions "initially focus" on smoking cessation, to improve the likelihood that a woman will conceive within a year. --D. Hollander
1. Bolumar F et al., Body mass index and delayed conception: a European multicenter study on infertility and subfecundity, American Journal of Epidemiology, 2000, 151(11):1072-1079.