Obesity was already known to be a risk factor for anovulation - an absence of ovulation. Now, a new study has revealed that it also reduces the chance of spontaneous pregnancy in women who are subfertile but ovulating normally.
The study looked at the relationship between body mass index (BMI) and the probability of pregnancy in women with no obvious fertility problems and who had been trying, unsuccessfully, to fall pregnant for a year.
It was found that for every BMI unit above 29 kg/m2, women were four per cent less likely to conceive as compared to women with a BMI between 21-29 kg/m2. Very obese women (BMI 35-40) had a probability of pregnancy that was between 26 and 43 per cent lower than women with a BMI between 21-29.
"This reduction in fertility is comparable to the increment of one year in female age. This study tells us that not only obese women with anovulation have lower chances of conception, but also obese women with a regular cycle. Given the increased prevalence of obesity, this is a worrying finding," said Dr Jan Willem van der Steeg, a medical researcher in obstetrics and gynaecology at the Academic Medical Center, Amsterdam, The Netherlands, and the lead author of the study.
As women in developed countries are tending to have their babies later in life, a woman aged 30 or over, who is also obese, would have an even greater reduction in her chances of becoming pregnant.
Dr van der Steeg and his 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 into account at the beginning of the study and the couples were followed until pregnancy or the start of fertility treatment within 12 months.
They found that obese women had a statistically significant lower probability of spontaneous pregnancy.
The researchers have suggested that a possible explanation of 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. 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 said.
Dr van der Steeg believes that obesity is a factor in the increasing numbers of couples seeking fertility treatments.
"Nowadays, one out of six couples will deal with subfertility once in their life. Adult obesity levels have increased four-fold over the last 25 years, with two-thirds of adults deemed overweight," he said.
"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," he added.
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