Couples who are both obese may have a harder time conceiving a child than couples who are both at a healthy weight, researchers reported Friday.
And obese couples struggle more with fertility than couples where only one person is obese, the team at the National Institutes of Health found.
Women who are too fat or too thin have long been told they may find it harder to conceive, but this new study, published in the journal Human Reproduction, shows the effect is far stronger when both the man and the woman are obese.
"Overall, obese couples were found to have approximately half the fecundability as couples with normal BMI," Rajeshwari Sundaram of the National Institute of Child Health and Human Development and colleagues wrote.
Fecundability is a calculation of how likely a couple is to conceive each menstrual cycle while trying for pregnancy.
"A lot of studies on fertility and body composition have focused on the female partner, but our findings underscore the importance of including both partners," Sundaram said in a statement.
More than a third of Americans are obese - defined as having a body mass index of 30 or higher.
In the NIH study, 500 couples in Michigan and Texas agreed to be studied as they tried to conceive.
"Overall, 27 percent of women and 41 percent of men were found to be obese," the team wrote
"Couples were followed daily for up to a year of trying," the team added.
When the woman was overweight or obese, the couple typically took longer to conceive, the researchers found. But the effect was marked when both the man and the woman were more than a little obese - with a BMI of 35 or higher. They were 60 percent less fecund than slimmer couples.
The team did not look for a biological explanation of why, but obesity can cause a wide variety of health issues.
Obese people often have higher levels of inflammation in their bodies. Fat cells also produce hormones, which may interfere with the hormones involved in conception.
In 2014, 35 percent of U.S. men and 40 percent of women were obese.
More than 5 percent of men and nearly 10 percent of women were morbidly obese, with a BMI of 40 or more. They're at much higher risk of related diseases.
Someone who is 5-foot-5 and weighs 149 pounds has a body mass index of 24, considered a healthy weight. Add a pound and the same person has a BMI of 25 and is considered overweight. At 180 pounds this person has a BMI of 30 and is considered obese.
The National Institutes of Health has a BMI calculator online here.