An internet-based prospective study of body size and time-to-pregnancy

Abstract

Background: Recent studies have shown that both female and male obesity may delay time-to-pregnancy (TTP). Little is known about central adiposity or weight gain and fecundability in women.

Methods: We examined the association between anthropometric factors and TTP among 1651 Danish women participating in an internet-based prospective cohort study of pregnancy planners (2007-2008). We categorized body mass index (BMI = kg/m(2)) as underweight (<20), normal weight (20-24), overweight (25-29), obese (30-34) and very obese (> or =35). We used discrete-time Cox regression to estimate fecundability ratios (FRs) and 95% confidence intervals (CI), controlling for potential confounders.

Results: We found longer TTPs for overweight (FR = 0.83, 95% CI = 0.70-1.00), obese (FR = 0.75, 95% CI = 0.58-0.97), and very obese (FR = 0.61, 95% CI = 0.42-0.88) women, compared with normal weight women. After further control for waist circumference, FRs for overweight, obese, and very obese women were 0.72 (95% CI = 0.58-0.90), 0.60 (95% CI = 0.42-0.85) and 0.48 (95% CI = 0.31-0.74), respectively. Underweight was associated with reduced fecundability among nulliparous women (FR = 0.82, 95% CI = 0.63-1.06) and increased fecundability among parous women (FR = 1.61, 95% CI = 1.08-2.39). Male BMI was not materially associated with TTP after control for female BMI. Compared with women who maintained a stable weight since age 17 (-5 to 4 kg), women who gained > or =15 kg had longer TTPs (FR = 0.72, 95% CI = 0.59-0.88) after adjustment for BMI at age 17. Associations of waist circumference and waist-to-hip ratio with TTP depended on adjustment for female BMI: null associations were observed before adjustment for BMI and weakly positive associations were observed after adjustment for BMI.

Conclusions: Our results confirm previous studies showing reduced fertility in overweight and obese women. The association between underweight and fecundability varied by parity.

Figure 1

Relation between female BMI and fecundability, by parity status at baseline, fitted by restricted cubic splines. Reference level for fecundability ratio is a BMI of 22 kg/m². The dotted line segments represent ranges beyond the last knot used in the spline fitting, denoting tail ranges that have less information. The curves are adjusted for age, partner's age, cycle regularity, cycle length, partner's BMI, physical activity, smoking, alcohol intake, intercourse frequency and waist circumference.