Fertility and Sterility 2009
Two newly published studies have shed new light on how obesity may affect aspects of fertility, suggesting a link between adolescent obesity and lifetime nulliparity, and demonstrating that female obesity impairs the outcome of IVF without affecting embryo quality.
Both studies are due to be published in the journal Fertility and Sterility. In the first study, researchers from centers in Newark, New York, and Oakland, California, USA (Polotsky AJ et al,) assessed the relationship between self-reported high-school body mass index (BMI) and lifetime nulliparity in a community based sample of 3,154 women aged 42-52 years. The data had been collected as part of the large Study of Women’s Health Across the Nation (SWAN).
Overall, 527 of the women (16.7 percent) were nulliparous. Analysis by self-reported BMI in high-school revealed an increase in lifetime nulliparity with increasing BMI, as follows:
- BMI in high-school below 18.5 kg/m2 (underweight): lifetime nulliparity 12.7 percent.
- BMI 18.5-24.9 kg/m2 (normal weight): 16.7 percent.
- BMI 25-29.9 kg/m2 (overweight): 19.2 percent.
- BMI 30 kg/m2 or higher (obesity): 30.9 percent.
The researchers found by multivariable logistic regression analysis (adjusting for potentially confounding factors such as adult BMI, ethnicity, and history of nongestational amenorrhea) that women who were obese during adolescence were significantly more likely to remain nulliparous compared with women who were of normal weight, with an odds ratio of 2.84 (95 percent confidence interval 1.59-5.10).
Adolescent obesity was also significantly associated with lifetime nulligravidity, with an odds ratio of 3.93 (95 percent confidence interval 2.12-7.26).
The researchers say the cross-sectional nature of the study suggests that the findings should be considered as useful for generating hypotheses (such as ‘does adolescent obesity cause diminished fertility?’) rather than as answers in themselves.
In the second study, researchers from the University of Valencia, Spain (Bellver J et al), conducted a retrospective study of data on all IVF-ICSI cycles performed at a single institution between January 2001 and April 2007. The total of 6,500 cycles were divided into four groups according to the women’s body mass index (lean, normal, overweight, and obesity).
The researchers found no significant difference between the four groups in the insemination procedure, the fertilization rate, the day of embryo transfer, the mean number of transferred and cryopreserved embryos, the percentage of blastocyst transfers, and embryo quality on day 2 and 3. There were, however, differences between groups in implantation, pregnancy, and live birth rates.
Pregnancy rates fell significantly with increased BMI, with an odds ratio of 0.984 (95 percent confidence interval 0.972-0.997) for each extra unit of BMI. Similarly, live birth rates fell significantly with increased BMI, with an odds ratio of 0.981 (95 percent confidence interval 0.967-0.995) for each extra unit of BMI.