Dr Jan Willem van der Steeg, a medical researcher and resident in obstetrics and gynaecology at the Academic Medical Center, Amsterdam, The Netherlands, and colleagues investigated the effects of obesity on spontaneous pregnancy in 3,029 subfertile couples between 2002 and 2004 in 24 hospitals in The Netherlands. The women had to be ovulating and have at least one, correctly functioning fallopian tube; the men had to have a normal semen analysis. A fertility history and other details, including height, weight and smoking habits, were taken at the start of the study and the couples were followed until pregnancy or the start of fertility treatment within 12 months.
In this study 3.7 per cent of women had a BMI below 18.5, 67 per cent had a BMI between 18.5-25, 19 per cent had a BMI between 25-30, 6.7 per cent had a BMI between 30-35, and 3.8 per cent had a BMI of 35 or more. The researchers used the women with a BMI between 21 and 29 as their reference group and found that BMI above 29 was associated with a statistically significant lower probability of spontaneous pregnancy than the reference group.
“In the case of a woman with a BMI of 35, the probability of spontaneous pregnancy was 26 per cent lower, and in the case of a woman with a BMI of 40 it was 43 per cent lower compared to the reference group,” said Dr van der Steeg.
The researchers believe that a possible mechanism to explain the relationship between BMI and pregnancy probability is the hormone leptin that regulates appetite and energy expenditure and is secreted by fatty tissues. “It is possible that obese women may have disturbed hormone levels, which decrease the chances of successful fertilisation and implantation,” said Dr van der Steeg. “The level of leptin in the body is positively related with the amount of fat in individuals without any mutations in their leptin regulation genes. Leptin levels are increased in obese people, suggesting a relative resistance to leptin. It is unclear whether or not obesity causes a resistance to leptin or vice versa. There is evidence that leptin may influence the process of steroid production by the ovaries.”
Dr van der Steeg continued: “We think that women should be informed about their lower pregnancy chances due to their overweight. Although this study does not show whether women’s chances of conceiving rise if they lose weight, because of the size of the cohort, with comparable women in different weight groups, we hypothesise that losing weight will increase the chance to conceive without treatment. Therefore, we would advise women to lose weight. Although the effect on better fertility is not proven, it is shown that the chance of a serious complication during pregnancy and labour is reduced.”
MEDICA.de; Source: European Society for Human Reproduction and Embryology