Can BMI affect fertility?

Having a body mass index (BMI) that is higher or lower than normal can affect many aspects of health, including reproductive health. Being underweight, as well as being overweight or obese, can affect fertility. Obesity in particular has been linked to increased risk of difficulty getting pregnant.

Endocrinologist Dr. Sheila Laredo, chief of staff at Women’s College Hospital, explains that the BMI associated with optimum fertility lies in the middle of the normal, healthy weight range.

“That is a body mass index of around 22 or 23,” she says. “When women become very underweight there is increased difficulty getting pregnant, and when weight rises there’s increased difficulty.”

The increased risk for trouble conceiving isn’t substantial for women who are modestly overweight, but it climbs with increased BMI, Dr. Laredo says. There is a marked increase in the risk for difficulty conceiving in women who are obese.

Irregular cycles

Part of the reason BMI is linked to fertility is that women who are overweight or obese, as well as those who are underweight, have a higher likelihood of having irregular periods, Dr. Laredo explains.

“Having irregular periods means you’re not ovulating – or producing an egg – every month. And obviously if you’re not producing an egg it’s harder to get pregnant,” she says. “If you’re not having a period every month, it’s harder to time intercourse in order to conceive.”

Even in those who do have regular cycles, women who are overweight or obese are still more likely to have difficulty getting pregnant than women with BMIs in the normal range, Dr. Laredo says. She adds that insulin resistance – which is more common in people with higher BMIs – may play a role, but researchers don’t know exactly how.

Practical goals

Research has shown how challenging it is to lose large amounts of weight and to keep it off. Suggesting that women lose 50 pounds before trying to conceive is probably not realistic, so Dr. Laredo recommends more attainable goals.

“We do try to get women to lose five to 10 per cent of their body weight if they can, and if they can do it in a healthy way,” Dr. Laredo says. That modest weight loss may improve the odds of getting pregnant, especially in women with irregular periods, because the weight loss helps re-establish more normal cycles.

However, Dr. Laredo does not recommend rapid weight loss for women trying to conceive.

“We do want to be careful,” she says. “There is some data showing that if there is very rapid weight loss immediately before a woman gets pregnant, that the baby may be born small. So we want healthy, reasonable weight loss.”

Bariatric surgery has been shown to enable fast and safe weight loss. However, Dr. Laredo notes that women are advised to wait a minimum of one year, possibly two years, after the surgery before a pregnancy to have the best chances of a good outcome for mother and baby.