Objective: Our aim was to examine the association between body mass index at age 18 and subsequent primary ovulatory infertility.
Study design: A nested case-control study was conducted within a cohort of 116,678 female registered nurses residing in 14 U.S. states. Cases comprised 2527 married nulliparous nurses unable to become pregnant for at least 1 year because of ovulatory disorder; controls comprised 46,718 married parous nurses with no history of infertility. The risk of ovulatory infertility for women at different levels of body mass index at age 18 was compared with that for women whose body mass index at age 18 was 20 to 21.9 (median for the cohort). Logistic regression was used to adjust for age at infertility or first birth, year of birth, age at menarche, physical activity during ages 18 to 22, smoking at ages 15 to 19, ethnicity, alcohol use at ages 18 to 22, use of oral contraceptives before age 22, and diagnosis of diabetes mellitus.
Results: Multivariate relative risks for infertility were: 1.2 (body mass index < 16), 1.1 (body mass index 16 to 17.9), 1.0 (body mass index 18 to 19.9), 1.0 (referent body mass index 20 to 21.9), 1.1 (body mass index 22 to 23.9), 1.3 (body mass index 24 to 25.9), 1.7 (body mass index 26 to 27.9), 2.4 (body mass index 28 to 29.9), 2.7 (body mass index 30 to 31.9), and 2.7 (body mass index > or = 32). The relative risks for all categories of body mass index above 23.9 were statistically significantly elevated. Greater body mass index at age 18 was a predictor of ovulatory infertility in women with and without a diagnosis of polycystic ovary syndrome.
Conclusion: These findings suggest that elevated body mass index at age 18, even at levels lower than those considered to be obese, is a risk factor for subsequent ovulatory infertility.