If one or both partners in a couple are overweight they are likely to have difficulty conceiving, a study published in the journal Human Reproduction has found.

The research, conducted in Denmark, surveyed 47,835 couples between 1996 and 2002 and found that obese couples were almost three times more likely to have trouble conceiving than couples of a normal weight, according to World Health Organisation standards. If one or both of the couple was overweight (rather than obese), the study found that it was likely to take them 1.4 times longer to get pregnant. Sub-fertility is defined as having to try for longer than 12 months to become pregnant.

Cecilia Ramlau-Hansen, of Aarhus University Hospital in Denmark and University of California Los Angeles, who led the study, advised couples who were overweight and having difficulty conceiving to try and lose some weight as the findings indicated a causal link between body mass index (BMI) and fertility. She stated that, 'Our results indicate that being overweight and obesity is a cause of sub-fertility, but from this study we cannot say that for sure', and allowed that there may be other underlying causal factors. However, earlier research has indicated that weight can affect fertility in both men and women. Ramlau-Hansen 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 scientists found on average that every kilogram of weight added an extra 2.84 days to the length of time taken to conceive, where the woman's BMI was 18.5 or more. The research did not look at whether obese couples had sex less frequently than normal weight pairs, or whether sterility occurred more frequently in obese or overweight couples.