Compared with planners, a higher proportion of non-planners reported in utero exposure to tobacco smoke (Table ). Planners tended to be older than those who had not planned their pregnancies. Sixty-two percent of planners had completed college or university, versus 41% among non-planners. A lower proportion of planners (27%) were exposed to active or passive smoking before the pregnancy than non-planners (36%). The two groups did not differ substantially on other variables (data not shown).
Among planners, the proportion with TTP > 12 months was higher among women with in utero exposure to tobacco smoke (12%) than in those without (11%) (Table ). The average age in the two groups was similar. Compared with unexposed subjects, women exposed prenatally to tobacco smoke were less likely to have completed college or university, and a higher proportion of them were exposed to active or passive smoking.
Characteristics of planners with and without in utero exposure to tobacco smoking.
The unadjusted FOR for exposure to in utero tobacco smoke among all subjects was 0.93 [95% confidence interval (CI): 0.91, 0.96), and the adjusted FOR was 0.96 (95% CI: 0.93, 0.98) (Table ). We saw no effect modification between a subject's adult tobacco smoking history before pregnancy and in utero tobacco smoke exposure (likelihood ratio test, P = 0.65, 3 degrees of freedom). Regardless of the non-significance of the effect modification test, we stratified the analysis by the subject's adult tobacco smoking before pregnancy (two categories: no adult tobacco exposure, with adult tobacco exposure including passive smoking) because subjects exposed in utero had higher proportions of exposure to adult tobacco smoking. The association among those exposed in utero but without any adult exposure would be less likely to be biased by residual confounding by adult smoking. The adjusted FORs for those who had no adult tobacco smoking exposure was 0.96 (95% CI: 0.93, 0.99) and for those who had adult tobacco smoking exposure it was 0.95 (95% CI: 0.91, 0.99).
Fecundability odds ratios (FORs) associated with in utero exposure to tobacco smoking (yes versus no) before and after probabilistic sensitivity analysis.
After taking into account exposure misclassification, the FOR for all subjects was 0.92 (95% CI: 0.88, 0.95), for subjects without adult tobacco smoking exposure it was 0.94 (95% CI: 0.89, 0.98) and for subjects who had a history of such exposure it was 0.91 (95% CI: 0.86, 0.95) (Table ). The association also became more pronounced after taking into account outcome (TTP) misclassification.
The odds of contraceptive failure were unrelated to in utero exposure to tobacco smoke (data not shown). The association between TTP and in utero exposure to tobacco smoking was slightly attenuated when we included non-planners in the analysis [e.g. adjusted FOR = 0.97 (95% CI: 0.95, 0.99), after imputation of non-planners' TTP as 1 month]. The other sensitivity analyses also indicated that the results were robust. For example, the estimated association was virtually the same when we restricted the analyses to first pregnancies (not shown), and when we assigned the 7506 women who provided an uncertain answer about in utero exposure to tobacco smoke as exposed or unexposed (data not shown). Using different censoring thresholds or adjusting for additional variables did not change the association (not shown).