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 womans 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 mens BMI is
Contact: Emma Mason
European Society for Human Reproduction and Embryology