Azara Blog: Overweight women allegedly less likely to get pregnant

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Date published: 2007/12/12

The BBC says:

An overweight woman's chance of getting pregnant steadily falls as her weight increases, a major study has found.

Among 3,000 women with fertility problems, there was a 4% drop in the chance of pregnancy for every body mass unit (BMI) rise above a certain point.

Dutch scientists, writing for the journal Human Reproduction, said that very obese women fared the worst.
...
The standard unit of weight is body mass index (BMI), which is the weight in kilograms divided by the height squared.

Anything above 25 is considered overweight, while exceeding 30 is defined as "obese".

All the women in the study had come to see fertility doctors, but there was no obvious reason for their failure to conceive, as they were still ovulating normally.

Some women with "unexplained infertility" do go on to become pregnant naturally.

The study found that there was a clear relationship between their BMI and their likelihood of achieving this.

Compared with women with a BMI between 21 and 29, for every BMI point between 30 and 35 there was a 4% drop in conception rates.

Severely obese women, with a BMI of over 35, were between 26% and 49% less likely to conceive compared with a BMI between 21 and 29.

Dr Jan Willem van der Steeg, who led the study, said: "Given the increased prevalence of obesity, this is a worrying finding.

"We think that women should be informed about their lower pregnancy chances due to their overweight.

"We hypothesise that losing weight will increase the chance to conceive without treatment."

This position is shared by the British Fertility Society, which issued guidelines to its members last month urging them to withhold fertility treatment from obese women until they lost weight.

Mr Tony Rutherford, who helped draw up the guidance, said: "Over the last few years there has been evidence that obesity can harm the fertility of women.

"It decreases the chances of getting pregnant, and increases the risks of pregnancy - to both mother and child.

A classic confusion between correlation and causation. So is it the weight that is causing the problem, or is their some other (no doubt complicated) factor which is causing both the increased weight and the many other correlations observed between weight and health. Van der Steeg does indirectly mention the one way to really check whether what they have observed is a causation rather than just a correlation. Take two random groups of obese women, make sure (somehow) that one of the groups loses weight and the other does not, and see what happens with pregnancy. Well, even here this might just show a correlation, because people who are not overweight might be more likely to have sex, so all of that, and other factors, would have to be controlled for as well. This study will never be done.

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