If both partners in a couple are overweight or obese, they are more likely to have to wait longer before successfully conceiving a child, according to new research published online in Europe's leading reproductive medicine journal, Human Reproduction, today (Wednesday 7 March). 
Researchers in Denmark studied 47,835 Danish couples between 1996 and 2002 and found that if both partners were obese the chances of the couple having to wait for more than a year before the woman became pregnant were nearly three times higher (2.74) than for a normal weight couple. If both partners were overweight, the likelihood they would have to wait longer than a year was 1.4 times higher. 
Cecilia Ramlau-Hansen, who led the study, warned that if further research supported the findings of the study that too much weight could make couples less fertile, this would have major implications for population levels, particularly in parts of the world where obesity and low fertility were, or would become, more common.
"If overweight and obesity actually is a cause of subfecundity, and if the obesity epidemic continues, this reduced capacity to reproduce could become a serious public health problem. Further research in this area is needed," she said.
Sub-fertility (or subfecundity) is normally defined as a waiting time to pregnancy of more than 12 months from the time a couple starts to have unprotected sex with an intention to conceive.
Earlier research has already shown that weight can affect fertility in both women and men, individually. However, this is the first study to look at the effect on fertility of overweight and obesity in couples. Ms Ramlau-Hansen, a doctoral student in the Department of Occupational Medicine, Aarhus University Hospital, Denmark, who is on a visiting scholarship to the University of California Los Angeles School of Public Health, USA, said: "Since it is quite common to have couples where both partners are overweight or obese, it is important to have risk estimates for couples rather than individuals."
The researchers obtained data from the Danish National Birth Cohort, a nationwide study of pregnant women and their offspring, which enrolled more than 100,000 women between 1996 and 2002. The women completed four interviews over a period of two years, giving information for both themselves and their partners on weight, height, previous pregnancies, smoking habits and socioeconomic groups. After excluding all couples where the information was incomplete, where a disease might be affecting the woman's weight and fertility, or where donated sperm was used, the researchers were left with data on 47,835 couples.
Ms Ramlau-Hansen said: "In this cohort, we had 6.8% obese men, 8.2% obese women and 1.4% couples where both were obese. The percentages of persons with normal weight were 53% for men and 68% for women.
"We found there was a relationship between the BMI of the couple and their fertility among the men and women in different BMI combinations. We found a higher risk of sub-fertility related to overweight and obesity for both men and women, particularly for couples where both were overweight. Underweight combined with obese partners, especially underweight men, seemed to cause additional pregnancy delays. The combination of an underweight man and obese woman was associated with a risk of sub-fertility nearly four times higher (3.79) compared to a normal weight couple, although we have to treat this figure with caution as there were only 22 couples in this category."
The researchers also looked more closely at 2,374 couples where the women had had more than one pregnancy, and where the women's initial BMI was 18.5 or more. They converted the women's waiting times to pregnancy into days and found that on average, every 1kg gained in weight added an extra 2.84 days to the waiting time. A further analysis of this group found that among 365 couples where the woman was overweight or obese before her first pregnancy (BMI of 25 and above) and either lost weight or maintained the same weight in the time up to the next pregnancy, for every 1kg of weight lost, the waiting time to pregnancy reduced by an average of 5.5 days.
"These findings indicate a causal association between BMI and fertility," said Ms Ramlau-Hansen. "In addition, for underweight women in this group, we saw a tendency for the time to pregnancy to decrease if the women gained weight, when compared to the first pregnancy. This is in line with earlier studies that show that being underweight can adversely affect a woman's fertility."
Since the study only included couples where the woman had become pregnant, it could not be used to detect an association between high BMI and sterility. "We believe that if BMI is a cause of sub-fertility, the effect is quantitative rather than qualitative - in other words, that couples have to wait longer to conceive the heavier they become, but there is not a threshold weight beyond which they become sterile. This is supported by all existing studies. A high BMI probably leads to sterility only in people with additional fertility problems," said Ms Ramlau-Hansen.
She continued: "Our results indicate that overweight and obesity is a cause of sub-fertility, but from this study we cannot say that for sure. There might be an unknown underlying factor, such as a disease or genetic factors, that causes both the overweight/obesity and the sub-fertility, and this could only be established in a randomised trial. A randomised trial could also establish whether losing weight might bring fertility back to normal values for overweight and obese couples. Our study indicates that it may be the case. In the meantime, our advice to overweight and obese couples that want to have a child would be that losing weight might decrease their waiting time to pregnancy."
Previous studies have shown that men's BMI is associated with semen quality and levels of reproductive hormones, and that, in women, obesity can adversely affect ovulation, conception, implantation and early foetal development. "Since reliable data on frequency and timing of sexual intercourse were not available, we cannot exclude the possibility that infrequent intercourse has delayed conception in overweight and obese couples," added Ms Ramlau-Hansen.
 Subfecundity in overweight and obese couples. Human Reproduction. doi:10.1093/humrep/dem035
 WHO criteria classifies people with a body mass index (BMI) of less than 18.5 as underweight, a BMI of 18.5-24.9 as normal weight, a BMI of 25-29.9 as overweight, and a BMI of 30 or more as obese.