Young black women who are obese or heavy through the hips were less likely to become pregnant, according to a substudy of the ongoing, prospective Black Women’s Health Study. Fecundity was significantly reduced in a dose-response fashion for women who were overweight (fecundity ratio, 0.89), obese (FR, 0.75) and very obese (FR, 0.68) after adjustment for age, education, smoking history, alcohol intake, physical activity, parity, region, and waist-to-hip ratio. A large waist-to-hip ratio (defined as 0.8 or greater), also was significantly associated with lower fecundity (FR, 0.73), with fecundity ratios less than 1 indicating reduced fecundity or longer time to pregnancy (TTP).
“Overall and central adiposity are associated with reduced fecundability in black women,” Lauren Wise, Sc.D., said at the annual meeting of the Society for Pediatric and Perinatal Epidemiologic Research. The substudy is the first TTP study in black women, and its results largely agree with previous studies consistently linking high body mass index and reduced fertility in white women.
Little is known about the determinants of fertility in black women, who are disproportionately affected by the obesity epidemic in the United States. Studies of central adiposity and fertility in whites have been inconclusive, with some suggesting that adiposity may interfere with estrogen metabolism, increase insulin resistance, and change the quality and pH of cervical mucus, said Dr. Wise of the Slone Epidemiology Center at Boston University.
The substudy cohort was drawn from 59,000 women in the Black Women’s Health Study, the largest study of U.S. black women’s health yet conducted and now in its 17th year of follow-up. A total of 15,500 women completed a Web-based survey in 2011 reporting the TTP for each planned pregnancy. There were 10,272 births, of which only 4,315 births (43%) were planned. The researchers excluded both the unplanned pregnancies and women who had incomplete data, a history of infertility, and age older than 40 years either in 1995 or while they attempted pregnancy; the final sample included 2,084 births and 209 unsuccessful pregnancy attempts among 1,706 women, aged 21-40 years.
The average age was 34 years for all BMI groups including those classified as overweight (BMI, 25-29 kg/m2), obese (BMI, 30-34) and very obese (BMI, 35 or greater). BMI was inversely associated with education and vigorous exercise, and was positively associated with waist-to-hip ratio, waist circumference, and current smoking status, reported Dr. Wise and her colleagues. After adjusting for all previous covariates plus BMI, researchers found that a waist circumference of 33-35 inches – but not beyond – was significantly associated with lower fecundity.
Fecundity was not lower in women who were underweight (BMI less than 18.5; FR, 1.11). Indeed, a recent systematic review involving 14 studies and 9,779 men reported that overweight and obese men are at increased risk of oligozoospermia or azoospermia, compared with normal-weight men (Arch. Intern. Med. 2012;172:440-2). Possible hypotheses for this relationship include hypogonadotropic hyperestrogenic hypogonadism due to aromatization of steroids in estrogens in peripheral tissue; direct alterations of spermatogenesis and Sertoli cell function; hip, abdominal and scrotal fat-tissue accumulation, leading to increased scrotal temperature; and accumulation of toxins and liposoluble endocrine disruptors in fatty tissue.