Fertility Impaired in Overweight and Obese Couples

AARHUS, Denmark, March 7 -- If would-be parents are fat, pregnancy may be a long time coming, according to a Danish study.

In a study of almost 48,000 Danish couples, researchers found that if both partners were obese, the chance of the couple having to wait more than a year before the woman became pregnant were almost three times higher than for a trim couple.

If both partners were merely overweight, the likelihood of waiting more than a year was 1.4 times higher, according to a study reported online in the March issue of Human Reproduction.

Earlier research had shown that weight can affect fertility in both men and women individually. However, fecundity is a couple concept, and this is the first study to look at the effect on fertility of overweight and obesity in couples, said Cecilia Ramlau-Hansen, a doctoral student at Aarhus University Hospital here, now on a visiting scholarship at UCLA, and colleagues in Denmark.

The data came from interviews, including weight measures and socioeconomic factors, with more than 64,000 pregnant women enrolled in the Danish National Birth Cohort from 1996 to 2000. After eliminating couples with incomplete information or where other factors warranted exclusion, the researchers were left with data on 47,835 couples.

In this cohort 6.8% of the men were obese, 8.2% of the women were obese, and both were obese in 1.4% couples.

Normal weight was defined as BMI 18.50 to 24.99 kg/m²; overweight as BMI 25 to 29.99 kg/m²; obesity as BMI 30 kg/ m². Underweight was BMI 2.

Among men and women with a BMI of 18.5 kg/ m² or more, there was a dose-response relationship between increasing BMI group and subfecundity (time to pregnancy of more than 12 months) as:

• Odds ratio (OR) = 1.32 (95% CI: 1.26-1.37) for women;
• OR = 1.19 (95% CI: 1.14-1.24) for men.

This higher risk of subfertility found in these men and women, was particularly true for couples, the researchers reported.

Among couples where both were either overweight or obese, the adjusted ORs for subfecundity were 1.41 (CI: 1.28 to 1.56) and 2.74 (CI: 2.27 to 3.30), respectively, compared with couples where both members were of normal weight.

Repeating the analysis using a six month time to pregnancy as the cutoff, gave similar results, as did restricting the analysis to first pregnancies only.

An underweight person, especially an underweight man, combined with an obese partner, seemed to cause additional pregnancy delays, the researchers noted.

The combination of an underweight man and an obese woman compared with a normal-weight couple, was associated with a risk of subfertility nearly four times higher (3.79) compared with a normal-weight couple. However, these findings must be treated with caution, the researchers said, as there were only 22 couples fitting this description, and the reasons for weight loss could be confounded by disease.

The researchers also analyzed 2, 374 couples where the women had more than one pregnancy and where the woman's initial BMI was 18.5 kg/ m² or more. Converting the women's waiting times to days, they found that each kilogram increase in weight between the two pregnancies was associated with an extra 2.84 days to pregnancy (95% CI: 1.33-4.35).

A further analysis of these groups found that among 365 couples where the woman was overweight or obese before her first pregnancy and either lost weight or maintained the same weight in the time till the next pregnancy, for every 1 kg of weight lost, the waiting time to pregnancy was reduced by an average of 5.5 days.

These findings indicate a causal association between BMI and fertility, the researchers said. For the underweight women in the group, there was a tendency for the time to pregnancy to decrease if the women gained weight, a finding in line with earlier studies showing that being underweight has an adverse effect on a woman's fertility.

Because this study included only couples who became pregnant, an association between high BMI and sterility was not detectable. "We believe, however, that the effect of BMI is quantitative rather than qualitative, if it is a cause of subfecundity, and this is supported by all existing studies," the researchers said. A high BMI probably leads to sterility only in people in people with additional fertility problems, they added.

Previous studies have shown that men's BMI is associated with semen quality and levels of reproductive hormones and that in women, obesity can affect ovulation, conception, implantation, and early fetal development.

"Since reliable information on the frequency and timing of sexual intercourse was not available, we cannot know whether infrequent intercourse delayed conception in overweight and obese couples," the investigators said.

In addition, they said, that although these results indicate that overweight and obesity are a cause of subfertility, this study cannot confirm that. There might be unknown underlying factors, such as disease or genetic factors, that cause these results. Only a randomized trial can establish this, they wrote.

Whether losing weight restores fecundity is also best studied in a randomized trial, although this study indicates that this may be the case, the researchers concluded.