Presumptions and Facts About Fertility and Obesity

The PregnantIt’s standard advice. “Every woman is different, but studies show that for women who have overweight or obesity, losing weight raised their chances of getting pregnant,” says the U.S. Office on Women’s Health. There’s just one little problem with this advice. It’s based on a presumption. The “studies” they mention document a correlation – not a causal relationship between losing weight and getting pregnant. It turns out that the relationship between fertility and obesity is a little more complicated.

Clearly, obesity can have an effect of making it more difficult to conceive a child. But a recent study in PLOS Medicine suggests that glib advice to “lose weight if you want to get pregnant” might not be entirely helpful.

A Randomized Controlled Trial

This was a multi-center RCT of 379 women with unexplained infertility and obesity. Average BMI was 39. The study randomly assigned them to either a weight loss group or a physical activity control group. In the weight loss group, increased physical activity, meal replacements, and orlistat helped people lose 6.6 percent of their starting weight after 16 weeks. The group with only physical activity had no significant weight loss.

Both groups then received three cycles of standard infertility treatment.

The bottom line on this study is straightforward. The weight loss group improved their metabolic health significantly. The control group did not. But that made no difference for getting pregnant or having a healthy live birth. In the control group, 15 percent of the subjects had a healthy live birth. For the weight loss group, the number was 12 percent – no significant difference.

In other words, for women with obesity and infertility, modest weight loss might improve their health, but it doesn’t help the odds of getting pregnant and having a baby.

Presumptions and Facts

Researcher Daniel Haisenleder sums it up quite well:

“Weight loss improved metabolic health in these subjects. Unfortunately the changes seen did not improve fertility. Infertility within this population remains an important health issue, and will require further studies to address the problem in the future.”

It is a fact that obesity is a chronic disease with many effects on a person’s health. Reduced fertility is one of them. Even modest weight loss can help with many of these problems. But this study tells us that it may not help with fertility.

The story is different for bariatric surgery. Case control studies have shown improved fertility after surgery compared to matched patients who do not have surgery. Recent research also suggests that pregnancy outcomes may be better after bariatric surgery. Of course, though these are careful studies of surgery’s effects, they are not randomized controlled studies.

In sum, we do know that obesity has an effect on fertility, along with many other dimensions of health. Thanks to this latest research, we also know that modest weight loss might not reverse the effects of obesity on fertility. So telling women with obesity that losing a little weight will help them get pregnant might not be truthful.

“Conjecture is good,” says Dean David Allison of the IU School of Public Health in Bloomington. “But knowing is better.” Now we know a bit more about obesity and fertility. We also know that we have more to learn.

Click here for the study in PLOS Medicine, here and here for more on obesity, bariatric surgery and pregnancy.

The Pregnant, painting by Giorgio de Chirico / WikiArt

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March 16, 2022