Obesity Reduces Fecundity Regardless of Menstrual Cycle Regularity or Smoking Status
Introduction: While studies have linked obesity to reduced fecundity, some evidence suggests that this association may only pertain to women with irregular menstrual cycles or who smoke. This study was designed to further investigate the effects of obesity on fecundity.
Methods: Researchers analyzed data from 7,327 pregnant women enrolled in the Collaborative Perinatal Project from 1959 to 1965. All of the women had planned their pregnancies. At the first study visit (at a median of 16 weeks’ gestation) the women were asked how long it had taken to become pregnant. Information also was collected on prepregnancy weight and height, patient demographics, smoking status, and medical and reproductive history.
Results: Women who were overweight or obese had a reduced fecundity compared to women of optimal weight (odds ratio [OR], 0.92 and 0.82, respectively); these associations were found even among women with normal menstrual cycles and were not modified by smoking status or age. The impact of obesity on fecundity was most pronounced among women who were pregnant for the first time.
Conclusion: Fecundity was reduced by 8% in overweight women and 18% in obese women. The effect of obesity on fecundity was found in all subgroups of women including those with regular menstrual cycles and nonsmokers. The authors suggested that weight loss may improve fecundity in overweight and obese women.
Tamara L. Wexler, MD, PhD, is an endocrinologist specializing in reproductive and neuroendocrinology, and an Attending in Medicine, Massachusetts General Hospital, Boston, MA.
Media reports on the dangers of obesity abound, including its impact on the health of pregnant women and their children. In addition, there is evidence that obesity itself may make it more difficult to conceive. The exact reason is not clear, though it has been proposed that this association may be due an effect on menstrual cycles.1-3 In this study, Gesink Law and colleagues examine the relationship between overweight/obesity and fecundity, while controlling for known contributors to difficulty conceiving such as irregular menstrual cycles, age, and smoking.
This study of 7327 women from the Collaborative Perinatal Project represents a retrospective population analysis. The researchers included only women who reported planned pregnancies and could estimate time to pregnancy (excluding 50,368 of a possible 59,391), excluding subjects missing BMI or lifestyle data; in cases of multiple pregnancies, first pregnancy only was included. The authors reported outcomes as fecundability odds ratios (FORs), with FOR < 1 indicating decreased fecundity and/or increased time to conception—setting 3 months or less as a "normal" time to conception. The WHO thresholds for overweight (25.0–29.9 kg/m2) and obesity (³30.0 kg/m2) were used, without further subcategorization of degree of obesity.
It is beneficial to have such a large group studied; however, cohort enrollment occurred several decades ago, which may impact the generalizability of the results today. Initial study visits occurred at 16 weeks' gestation, so women who conceived but had early miscarriages were not included in the study (though the group did include women with a history of miscarriage). Much of the data was self-reported, including time to pregnancy and pre-pregnancy weight, as well as lifestyle factors. Alcohol and drug use information was not used, and information on caffeine and diet was not ascertained.
The study found that overweight and obesity have a negative impact on fecundity. The effect was most pronounced in time to first pregnancy (also noted in the Jacobsen et al study, reviewed in this EndoScan). The authors suggest that the impact of any given factor may be more visible in first pregnancy due to a wider range of measured fertility in nulliparous women.4
As this population analysis was limited to observation it was not designed to assess whether weight loss in obese women improves fecundity. A study examining the impact of weight loss, and method of weight loss (given, for example, a potential confounding role of high degrees of exercise), on conception would provide additional information. As the U.S. population of overweight/obesity remains high,5 it is important not only to elucidate the mechanism by which weight interferes with conception, but in particular to define measures to improve fecundity.
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2. Pasquali R, Pelusi C, Genghini S, Cacciari M, Gambineri A. Obesity and reproductive disorders in women. Hum Reprod Update. 2003;9(4):359-372.
3. Haslam DW and James WPT. Obesity. Lancet. 2005;366,1197–1209.
4. Axmon A, Rylander L, Albin M, Hagmar L. Factors affecting time to pregnancy. Hum Reprod. 2006;21(5):1279-1284.
5. National Center for Health Statistics. NCHS Data on Obesity. February 2012. http://www.cdc.gov/nchs/data/factsheets/factsheet_obesity.htm. Accessed May 5, 2014.