Obesity and Infertility: Impact of BMI on Outcomes of IVF

Link Between Body Weight and Fertility

The prevalence of obesity has tripled since 1975, and in 2016 it was estimated that 1.9 billion adults worldwide were overweight or obese.^[1] On the basis of 2013-2014 data, 66.9% of US women were overweight or obese.^[2] Obesity is best described by the body mass index (BMI), as weight in kg/height in meters squared. The four weight categories according to BMI are:

Underweight (BMI < 18.5 kg/m²)
Normal weight (BMI 18.5 kg/m²-24.9 kg/m²)
Overweight (BMI 25 kg/m²-29.9 kg/m²)
Obese (BMI ≥ 30 kg/m²)

Obesity carries numerous health risks and is associated with infertility as well. As the BMI increases, the risk for ovulatory dysfunction also increases, leading to infertility. Many obese women require assisted reproduction technology (ART). However, ART success rates have also been shown to be affected by BMI, although the results are conflicting.^[3,4]

Obesity and IVF Outcomes

A recent multicenter analysis^[5] studied the outcomes of in vitro fertilization (IVF) in women in different BMI categories and various infertility etiologies.

This retrospective analysis used data from 51,198 women who were undergoing their first autologous IVF cycle. Comparisons were made between underweight, normal weight, overweight, and obese class I (30 kg/m²-34.9 kg/m²) and obese class II-III (>35 kg/m²) BMI categories. Subgroup analysis based on infertility etiology (diminished ovarian reserve, male, endometriosis, uterine, unexplained, ovulatory, and polycystic ovarian syndrome) was performed.

Age, parity, infertility etiology, ovarian reserve markers, the number of embryos transferred, and the proportion of day 3 versus day 5 transfers were similar across the BMI groups.

IVF outcomes included the following:

The risk for cycle cancellation was higher in overweight and obese women whereas it was comparable in normal and underweight patients.
Fewer oocytes were collected from obese women compared with women in underweight, normal weight, or overweight categories.
Compared with normal weight women, those in the other BMI categories had fewer usable embryos.
Women in obese class I (adjusted odds ratio [aOR], 0.89; 95% confidence interval [CI], 0.83-0.95) and obese class II-III (aOR, 0.86; 95% CI, 0.79-0.93) had lower clinical pregnancy rates compared with normal weight women. Underweight and overweight women had comparable clinical pregnancy rates compared with normal weight women.

When subgroups were analyzed, obese women with a diagnosis of PCOS, male factor infertility, uterine factor infertility, or ovulatory dysfunction had more canceled cycles, fewer oocytes, and fewer usable embryos than women with the same infertility etiology but lower BMIs. Obese patients with PCOS had lower pregnancy rates compared with normal weight women with PCOS.

Viewpoint

The prevalence of obesity has increased dramatically in the past few decades, and the fertile population is no exception from this trend. Dietary habits and sedentary lifestyles are the most likely causes of obesity in women of childbearing age.

Obesity is accompanied by altered steroidogenesis, which influences the secretion of follicle-stimulating hormone and luteinizing hormone, leading to anovulation. Obesity also may interfere with implantation and has been associated with higher pregnancy loss rates. Obese men have lower sperm parameters, contributing to lower fertility rates. Therefore, women with high BMIs often require fertility treatment to become pregnant.

Previous reports have been conflicting on the full impact of higher BMI on ART outcomes.^[3,4] The need for more gonadotropins, more frequent cycle cancellation, and lower implantation/pregnancy rates have been all reported. Live birth rates, however, did not differ among these studies.

This analysis unfortunately did not report pregnancy loss and live birth rates. Only the first embryo transfers were included, so cumulative pregnancy rates could not be compared. It's possible, however, that with fewer oocytes collected and fewer available transferable embryos, the cumulative pregnancy and live birth rates would be even worse among obese women.

The inclusion of only first transfers does not permit a comparison of frozen embryo transfer (FET) outcomes among women in different body weight categories. Women who are obese need more gonadotropins and may have prolonged stimulations that could adversely influence endometrial receptivity. Success rates may be higher with FET cycles.

The impact of weight loss achieved through lifestyle changes, weight loss medications, or bariatric surgery also needs further evaluation. Finally, we need to advise patients individually on the best approach for the management of infertility. If treatment is withheld in an otherwise motivated patient, we may increase dropout from fertility care. However, patients with higher BMIs need to be counseled that their reproductive performance is likely to suffer from excess weight—a negative but potentially modifiable prognostic factor.

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