title: 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.
authors.affiliation: Department of Epidemiology, Boston University School of Public Health, Boston, MA, USA. lwise@bu.edu
authors.forename: Lauren A
authors.initials: LA
authors.lastname: Wise
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Rothman
authors.forename: Ellen M
authors.initials: EM
authors.lastname: Mikkelsen
authors.forename: Henrik Toft
authors.initials: HT
authors.lastname: Sørensen
authors.forename: Anders
authors.initials: A
authors.lastname: Riis
authors.forename: Elizabeth E
authors.initials: EE
authors.lastname: Hatch
mesh_terms: D000293:Adolescent; D000328:Adult; D050218:Body Fat Distribution; D015992:Body Mass Index; D049628:Body Size; D001835:Body Weight; D003718:Denmark; D005260:Female; D005298:Fertility; D006801:Humans; D020407:Internet; D008297:Male; D009765:Obesity / Q000150:complications*; D050177:Overweight; D010298:Parity; D011247:Pregnancy; D011446:Prospective Studies; D013851:Thinness / Q000150:complications; D013997:Time Factors; D055105:Waist Circumference; D049629:Waist-Hip Ratio