How exactly does obesity affect fertility?

By Dr. Eva Martin of Elm Tree Medical Inc.

The obesity epidemic is a popular topic in the media. Our news feeds are flush with articles about the dangers of obesity: hypertension, sleep apnea, cancer, dyslipidemia, and cardiovascular disease. In the OBGYN world, every month brings new journal articles about the dangers of obesity during pregnancy, everything from birth defects to obstetric complications to a higher risk of obesity in offspring. Recently, the American Society for Reproductive Medicine issued a committee opinion on the ways in which obesity affects fertility. So, how, exactly, does obesity affect fertility?

1. Lower chance of conceiving

Women with obesity are more likely to have menstrual cycle abnormalities and ovulatory dysfunction. Women with BMI>27 are 3.1 times more likely to have ovulatory dysfunction. There are many ways in which the physiology of obesity changes the body's reproductive system. High insulin interacts with sex hormones, and fat produces more estrogen, both of which adversely affect fertility. Fat tissue also produces substances called "adipokines" which directly interfere with the ovaries' functioning. Finally, women with obesity often have lower LH surges (the hormone that initiates ovulation). Overall, women with obesity have lower odds of becoming pregnant. After 1 year of stopping birth control, 81% of normal weight women will become pregnant. Only 66% of obese women will become pregnant after a year of trying to conceive.

2. Lower chance of success with IVF

For women with infertility, assisted reproductive technology (ART) often holds the solution. However, for women with obesity, interventions, such as IVF, are less likely to result in a live birth. Women with BMI>25 (overweight or obese) have a 10% lower live-birth rate after IVF than women with BMI in the normal range. For women undergoing IVF, obesity decreases the odds of a live birth by 37% for BMI 30-34, 61% for BMI 35-39, and 68% for BMI>40. There is a 2% decrease in live births after IVF for every 1 unit increase in BMI.

It is believed that embryo implantation is impaired in women with obesity. Eggs from women with BMI >25 are smaller and more likely to stop growing after fertilization with a sperm (arrest prior to blastulation). Studies have shown increased triglycerides in the embryos that stopped growing. The embryos from eggs from obese women that are implanted into the uterus during IVF (blastocysts) have fewer cells, less glucose consumption, more triglycerides, and altered protein metabolism.

3. Higher risk of miscarriage

In addition to a lower chance of success with IVF, obesity is also linked to a higher risk of miscarriage after IVF. Women with obesity have a 1.3X increased risk of miscarriage after IVF. Obesity is also linked to serious pregnancy complications, such as gestational diabetes, high blood pressure, and shoulder dystocia. Obesity is also linked to birth defects such as neural tube defects, cleft lip and palate, and heart anomalies. In these ways, obesity can affect pregnancy outcomes.

While all of this research is pretty grim, it is not without hope. Preconception weight loss is the best way to reduce these risks and correct obesity-related infertility. Guidelines recommend 150 minutes of exercise a week and restricting caloric intake. A diet of 1,000 to 1,200 calories a day can result in a 10% decrease in body weight in 6 months. Women who decide to undergo bariatric surgery should wait at least 1 year after surgery before attempting pregnancy. 

We want to hear from you! How do you counsel patients about obesity and fertility? What are your best tips for healthy weight loss?