Wildenschild, Cathrine2; Riis, Anders H7; Ehrenstein, Vera7; Heitmann, Berit L3; Hatch, Elizabeth E4; Wise, Lauren A5; Rothman, Kenneth J6; Sørensen, Henrik T7; Mikkelsen, Ellen M7
1 Department of Clinical Medicine - Department of Clinical Epidemiology, Department of Clinical Medicine, Health, Aarhus University2 Department of Clinical Epidemiology, Aarhus University Hospital, Aarhus, Denmark.3 Institute of Preventive Medicine, Bispebjerg and Frederiksberg Hospital, Copenhagen University and National Institute of Public Health, University of Southern Denmark, Copenhagen, Denmark; The Boden Institute of Obesity, Nutrition Exercise & Eating Disorders, University of Sydney, Sydney, New South Wales, Australia.4 Department of Epidemiology, Boston University School of Public Health, Boston, Massachusetts, United States of America.5 Department of Epidemiology, Boston University School of Public Health, Boston, Massachusetts, United States of America; Slone Epidemiology Center, Boston University, Boston, Massachusetts, United States of America.6 Department of Epidemiology, Boston University School of Public Health, Boston, Massachusetts, United States of America; RTI Health Solutions, Research Triangle Park, North Carolina, United States of America.7 Department of Clinical Medicine - Department of Clinical Epidemiology, Department of Clinical Medicine, Health, Aarhus University
a prospective cohort study
OBJECTIVE: To examine the association between a woman's birth weight and her subsequent fecundability. METHOD: In this prospective cohort study, we included 2,773 Danish pregnancy planners enrolled in the internet-based cohort study "Snart-Gravid", conducted during 2007-2012. Participants were 18-40 years old at study entry, attempting to conceive, and were not receiving fertility treatment. Data on weight at birth were obtained from the Danish Medical Birth Registry and categorized as <2,500 grams, 2,500-2,999 grams, 3,000-3,999 grams, and ≥ 4,000 grams. In additional analyses, birth weight was categorized according to z-scores for each gestational week at birth. Time-to-pregnancy measured in cycles was used to compute fecundability ratios (FR) and 95% confidence intervals (CI), using a proportional probabilities regression model. RESULTS: Relative to women with a birth weight of 3,000-3,999 grams, FRs adjusted for gestational age, year of birth, and maternal socio-demographic and medical factors were 0.99 (95% CI: 0.73;1.34), 0.99 (95% CI: 0.87;1.12), and 1.08 (95% CI: 0.94;1.24) for birth weight <2,500 grams, 2,500-2,999 grams, and ≥ 4,000 grams, respectively. Estimates remained unchanged after further adjustment for markers of the participant's mother's fecundability. We obtained similar results when we restricted to women who were born at term, and to women who had attempted to conceive for a maximum of 6 cycles before study entry. Results remained similar when we estimated FRs according to z-scores of birth weight. CONCLUSION: Our results indicate that birth weight appears not to be an important determinant of fecundability.