Being overweight, obese or underweight could affect a woman's chances of becoming pregnant and having a healthy pregnancy—and male fertility is also affected by those factors, according to a new Dutch study.
Scientists measured the body mass index—weight divided by height—of men and women in more than 3,600 couples, either while they were trying to get pregnant or early in the pregnancy, and compared this with their fertility and pregnancy outcomes.
The researchers found that a higher BMI in both women and men was associated with lower fertility, meaning it took longer for those couples to get pregnant. In general, the higher the BMI, the longer it took couples to conceive.
In the U.S., 1 in 5 married women aged 15 to 49 who have not given birth before experience infertility, according to estimates from the Centers for Disease Control and Prevention. However, nutritional therapist Julia Young, who specializes in fertility, told Newsweek that fertility is not just a woman's problem.
"It takes two to make a baby, [so] both partners share the responsibility when it comes to fertility and a healthy pregnancy," Young said.
"Often there is an emphasis on the woman's side because she is the one carrying the baby. However, this is only half of the equation; sperm health is equally significant," she added.
Young said that in 30 to 40 percent of cases, infertility is caused by the male partner, adding, "Fertility should be a shared journey, working together as a team and supporting each other."
In the study, overweight and obese women were found to be less likely to conceive within one month of trying, more likely to take more than 12 months to conceive, and more likely to experience miscarriages, compared to women in a healthy range.
Underweight women and overweight and obese men also experienced problems conceiving a child.
"When people are considering having children soon, it is important to critically review the BMI of both partners," Dr. Vincent Jaddoe, a professor and an author of the study, told Newsweek.
"When this is outside of the normal range, one could consider focusing on weight loss or gain first, before actively trying to conceive," he added. This could lower the risk of fertility problems and miscarriages.
The study authors found that it was not just the categories of "overweight" and "obesity" that were associated with fertility issues. For every unit increase of BMI from the healthy range to obesity, fertility in both men and women decreased.
"Even when women had a BMI in the normal range, overweight and obesity in men only was still associated with lower fecundability," Jaddoe said. "Fecundability" was defined in the study as the ability to conceive a child within the first month of trying. He added that the same was true for "subfertility," where fertility problems persisted for 12 months of more.
"This implies the independent effect of male BMI, but also the combined effect of female and male BMI on fertility outcomes," he said.
"We observed not only that obesity but also overweight and underweight were associated with fecundability, subfertility and miscarriage risk," Jaddoe continued.
The scientists proposed that these associations might be due to hormonal problems, which can be caused by an excess or lack of fat tissue among people with BMIs outside the healthy range.
This can lead to a decreased quality of eggs, uterine environments and sperm, Jaddoe said.
Young explained some of the mechanisms that can cause fertility problems in people with BMIs outside the healthy range.
"In women, a BMI that is too high can interfere with ovulation by disrupting hormonal balance, which can lead to irregular or even absent periods, making it harder to conceive and pinpoint when you are ovulating," she said.
"However, a BMI that is too low can lead to amenorrhea, where the menstrual cycle stops altogether, which also impacts fertility," the nutritional therapist added.
In men, Young said, being overweight or obese could affect sperm count and lower testosterone levels.
For pregnant women, maintaining a healthy weight "is essential for a healthy pregnancy," Young said, as overweight women can experience a higher risk of pregnancy complications, including gestational diabetes, high blood pressure and preeclampsia.
"Being underweight can also pose risks," she said, "such as giving birth to a low-birth-weight baby, which can result in developmental challenges and an increased risk of health issues later in life."
Both Young and Jaddoe emphasized the importance of achieving a healthy weight before trying to conceive, rather than trying to lose weight while pregnant.
"We do not recommend actively trying to lose weight during pregnancy, since this negatively influences the maternal nutrition status and thus embryo," Jaddoe said.
Young said there were a number of ways, aside from maintaining a healthy weight, that could help prospective parents improve their chances of delivering a healthy baby.
"It is important to focus on a diet rich in key nutrients that support reproductive health," she said, mentioning folate, which is found in leafy greens, legumes and grains; omega-3, found in salmon, flaxseeds and walnuts; and zinc, found in red meat, chickpeas and pumpkin seeds.
Young also said following a Mediterranean-style diet is "highly beneficial for fertility."
She added, "Both partners should aim to reduce processed foods, limit alcohol intake, and avoid excess sugar, which can negatively affect hormonal health and fertility."
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Reference
Bozem, A. J., Blaauwendraad, S. M., Mulders, A. G. M. G. J., Bekkers, E. L., Kruithof, C. J., Steegers, E. A. P., Gaillard, R. & Jaddoe, V. W. V. (2024). Preconception and Early-Pregnancy Body Mass Index in Women and Men, Time to Pregnancy, and Risk of Miscarriage. JAMA Network Open 7(9). doi.org/10.1001/jamanetworkopen.2024.36157
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