Can being overweight interfere with your chances of getting pregnant?

When Jane Greene* started trying to get pregnant, she went off the birth-control pill for the first time in years and was surprised when her periods became very irregular. “We tried for a year, and I only had three or four periods during that time,” she says.

Her family doctor referred her to an OB/GYN who put her on clomiphene, a pill that makes you ovulate more often. That turned out to be all she needed: the very first month after going on it, she got pregnant.

The doctors never said exactly why she wasn’t getting regular periods, but Jane suspects it was due to her obesity. After she had her daughter, she adopted a vegan diet and lost 20 pounds off her prepregnancy weight, and her period went back to normal. When she tried to conceive her second child at that weight, it took her only four months to get pregnant—and she didn’t need any drugs. “It seems like once my weight gets past a certain point, my body just doesn’t ovulate,” she says.

Research shows that obesity can impact fertility, and irregular cycles caused by the excess weight are the most common culprit. Though most people who are obese will get pregnant without any medical intervention, as weight increases, there’s a higher probability women might go longer between periods, says Neal Mahutte, chair of the Clinical Practice Guideline Committee for The Canadian Fertility and Andrology Society that created soon-to-be-released guidelines around obesity and infertility. “You’re twice as likely not to ovulate each month if you have obesity.” That gives you fewer chances to get pregnant every year, and it can make timing sex more difficult, because you might not know when you’re ovulating.

This less frequent ovulation is thought to happen because fat cells make estrogen, which disrupts the hormone that tells your body to release eggs. “Fat cells produce a weak estrogen called estrone, which can feed back to the central part in your brain that controls your normal cycles, and can essentially trick your brain into thinking that it’s not time to have another cycle,” says Gregory Davies, chair of maternal-fetal medicine at Queen's University.

This doesn’t seem to be an issue for people who are overweight—instead, it’s mainly a concern for those who are obese, which typically means having a BMI of more than 30. The risk increases the more obese someone is, and body shape matters, too, with abdominal fat potentially having a bigger impact on ovulation than fat elsewhere in the body. Women who are underweight are also at risk for irregular ovulation, most likely because their bodies are not producing enough of the hormone that’s necessary to release of a mature egg.

As Jane discovered, getting to a more moderate weight can reverse the issue. That’s why weight loss is the first thing doctors will recommend for obese women who are struggling with infrequent ovulation. The next option is usually medications like letrozole or clomiphene, which increase the levels of the hormone that stimulates your ovaries to release an egg, in order to encourage more regular ovulation. These drugs work for most women with obesity.

One of the reasons losing weight is the preferred choice for people with obesity is that, in addition to resolving fertility issues, it significantly lowers other risks that come with obesity during pregnancy. “If women can bring their BMI below 30, they’re more likely to get pregnant, less likely to miscarry, and their baby is more likely to be born at term and healthy,” says Beth Taylor, a fertility doctor and co-founder of the Olive Fertility Centre in Vancouver.

But doctors are are also aware of how hard losing weight can be. “Most of the patients who are obese who come to see us in fertility clinics have tried many diets, and seen multiple dietitians,” says Mahutte. “So when you ask, well, have you ever thought about dieting, their eyes roll.” And for women who are older, they may not have time to spend months trying to lose weight, since their fertility is declining at the same time.

The good news is, you don’t have go back to your high-school weight to solve the problem, since small weight loss can fix fertility issues. Losing five to 10 percent of body weight is often enough to make people begin ovulating regularly on their own, even if they remain obese. That’s all it took for Jane, who lost weight out of a desire to be healthier herself, and found that resolving her fertility issues was a happy side effect. “I was still in the obese category, but down about 10 percent or so when I started having regular periods again,” she says. “I’m confident that losing weight helped.”

*Name has been changed

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