March 7, 2007 -- A couple trying to conceive may face an extra challenge when both the man and the woman are overweight or obese, new research suggests.
Compared with normal-weight couples, obese couples participating in a Danish study were almost three times as likely to take more than a year to achieve a pregnancy.
The findings strongly suggest, but do not prove, a causal association between excess weight in both partners and decreased fertility, researcher Cecilia Ramlau-Hansen tells WebMD.
“Because of the study design we cannot say for a fact that it is extra body fat that makes people less fertile, but it certainly appears that this is the case,” she says. “If a couple is overweight and wants to have a child it may be beneficial for both partners to attempt weight loss.”
Weight Loss Reduced Time to Conception
The researchers analyzed data from 47,835 couples who participated in a nationwide study of pregnancy outcomes in Denmark. Women in the study completed four interviews over a period of two years, giving information for both themselves and their partners on weight, height, previous pregnancies, smoking, and socioeconomic status.
The findings are published in the March issue of the journal Human Reproduction.
A total of 8.2% of the women, 6.8% of the men, and 1.4% of the couples in the study were obese, defined as having a body mass index (BMI) of 30 or more. BMI looks at weight in relation to height and is used as an indicator of body fat.
As measured by BMI, a 5-foot-2-inch person who weighs 165 pounds or more is considered obese, as is a 6-foot-tall person who weighs 220 or more.
Just over half of the men and two-thirds of the women in the study were normal weight.
Ramlau-Hansen and colleagues from Denmark’s University of Aarhus evaluated the time it took the couples to become pregnant. Sub-fertility was defined as failure to conceive for at least a year after initiating unprotected sex with the goal of conceiving.
Obese women had a 78% greater risk of being sub-fertile than normal-weight women, and obese men had a 49% increased risk for sub-fertility than normal-weight men.
The risk of taking more than a year to achieve a pregnancy was 2.74 times higher when both partners were obese than for a normal-weight couple.
The researchers further examined 2,374 couples who had more than one pregnancy. When they converted the length of time that it took the women to get pregnant into days, they concluded that for overweight or obese women, every 2.2 pounds of weight loss reduced the time to conception by an average of 5.5 days.
Heavier Men Have Less Sex
The suggestion that weight loss seems to improve fertility for both women and, to a lesser extent, men has important potential public health implications, says epidemiologist Donna Baird, PhD, of the National Institute of Environmental Health Sciences (NIEHS).
Baird co-authored a 2006 NIEHS study that linked obesity to infertility in men. The researchers concluded that a 3-unit increase in BMI increased the risk of infertility by about 10%.
At least one other study has linked obesity in men to a decline in sperm quality, but Baird says more research is needed to confirm the association between body weight and infertility in men.
She adds that the decline in fertility among overweight and obese men may have more to do with sexual function than sperm quality.
“There are a lot of gaps in what we know,” she tells WebMD. “We didn’t have data on the frequency of sexual intercourse among men, and we know that obesity can certainly impact sexual function. Low libido and erectile dysfunction, for example, are much more common in obese men.”