Body mass index, physical activity and fecundability in a North American preconception cohort study.

OBJECTIVE: To evaluate the association between adiposity, physical activity (PA), and fecundability.
DESIGN: Prospective cohort study.
SETTING: Not applicable. PATIENT(S): A total of 2,062 female pregnancy planners from the United States and Canada who were enrolled during the preconception period. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): Self-reported pregnancy. Fecundability ratios (FRs) and 95% confidence intervals (CIs) were estimated using proportional probabilities models that adjusted for potential confounders. RESULT(S): Relative to body mass index (BMI) 18.5-24 kg/m(2), FRs for BMI <18.5, 25-29, 30-34, 35-39, 40-44, and ≥45 kg/m(2) were 1.05 (95% CI 0.76-1.46), 1.01 (95% CI 0.89-1.15), 0.98 (95% CI 0.82-1.18), 0.78 (95% CI 0.60-1.02), 0.61 (95% CI 0.42-0.88), and 0.42 (95% CI 0.23-0.76), respectively. Reduced fecundability was observed among women with the largest waist-to-hip ratios (≥0.85 vs. <0.75; FR = 0.87, 95% CI 0.74-1.01) and waist circumferences (≥36 vs. <26 inches [≥90 vs. <66 cm]; FR = 0.80, 95% CI 0.59-1.01). Tendency to gain weight in the chest/shoulders (FR = 0.63, 95% CI 0.36-1.08) and waist/stomach (FR = 0.90, 95% CI 0.79-1.02), relative to hips/thighs, was associated with lower fecundability. Moderate PA was associated with increased fecundability (≥5 vs. <1 h/wk; FR = 1.26, 95% CI 0.96-1.65), but there was no dose-response relation. Among overweight/obese women (BMI ≥25 kg/m(2)), fecundability was 27% higher for vigorous PA of ≥5 versus <1 h/wk (95% CI 1.02-1.57). CONCLUSION(S): Various measures of overall and central adiposity were associated with decreased fertility among pregnancy planners. Vigorous PA was associated with improved fertility among overweight and obese women only; moderate PA was associated with improved fertility among all women.