She ain’t heavy, she’s my mother

“Women who are seriously obese almost halve their chances of having a baby,” the Daily Express warned on December 12 2007. The BBC also covered the story and said a study has found that being overweight can reduce a woman’s chances of getting pregnant, and that women with a body mass index (BMI) of more than 35 are 26% to 49% less likely to get pregnant than women who have a BMI of 21 to 29.

These stories are based on a study of more than 3,000 couples who had trouble conceiving, which looked at whether the woman’s BMI affected her chances of getting pregnant naturally over the coming year.

This study does provide some evidence that being obese may reduce your chances of becoming pregnant. However, it is worth bearing in mind that there are many reasons why a couple may have trouble conceiving, and couples having trouble should consult their doctor who can give advice and refer them for further investigation as needed. Maintaining a healthy weight has benefits for everyone, not just women who want to have a baby.

Where did the story come from?

Dr Jan Willem van der Steeg and colleagues from the University Medical Centre Rotterdam, the Academic Medical Centre Amsterdam, and several Dutch obstetrics and gynaecology departments carried out this research. The study was funded by The Netherlands Organisation for Health Research and Development. The study was published in the peer-reviewed medical journal: Human Reproduction .

What kind of scientific study was this?

This prospective cohort study looked at whether obesity affects the likelihood of spontaneous conception in couples who are subfertile. The researchers particularly wanted to know whether obesity had an effect in women who were capable of ovulating, as obesity had already been shown to reduce fertility by reducing ovulation.

The researchers consecutively enrolled 3,029 couples who had been unsuccessfully trying for a pregnancy for at least a year. They had attended hospital clinics between January 2002 and February 2004. The couples were all given a "fertility work-up" which assessed them for possible reasons for their subfertility. This included fertility history, details about height and weight, and smoking habits. An examination of the woman's fallopian tubes was carried out, her ability to ovulate was assessed and an analysis was performed on the man’s semen.

The height and weight information was then used to calculate the BMI of each of the women.

Women included in the study had to be able to ovulate, and have at least one clear fallopian tube, and men had to have normal semen.

Using the information collected, doctors calculated the probability of the couple getting pregnant spontaneously in the next year. Based on this probability and using Dutch national guidelines, they were told if they qualified for treatment or if they should continue to try for a baby naturally.

Couples were followed for 12 months, or until they had treatment, to see if the women became pregnant without any medical intervention. The researchers compared the proportion of women who became pregnant in different BMI categories to see if they differed. Factors that might affect fertility were taken into account, including; the woman’s age, how long they had been trying for a baby, whether the woman had been pregnant before, how mobile the man’s sperm was, whether either partner smoked, and how they were referred to the clinic.

What were the results of the study?

Of the couples, 529 (about 17 per cent) became pregnant within a year without medical intervention.
 

The researchers found that the higher a woman’s calculated BMI was above 29, the less likely she was to become pregnant spontaneously. For every unit of BMI above 29, a woman was four per cent less likely to become pregnant than if she had a BMI between 21 and 29.

Having a BMI below 21 also reduced a woman’s chances of getting pregnant, but this difference was small and might have occurred by chance.

What interpretations did the researchers draw from these results?

The researchers concluded that obesity reduced the chances of getting pregnant spontaneously in subfertile women who are capable of ovulating.

What does the NHS Knowledge Service make of this study?

This was a relatively reliable study, which did have a number of limitations, some of which the authors acknowledge.

• Although the researchers did try to take into account factors other than BMI that could affect the chances of becoming pregnant, there are other factors that might affect the results; for example, how often the couples  had sex or the BMI of the man, neither of which were measured in this study. How often a woman had sex would also affect their chances of conceiving, and if this was influenced by their BMI, then this could skew the results in favour of the groups who were having intercourse more frequently. 
• Women reported their height and weight themselves, and they may have reported these incorrectly. As the BMI was calculated using these, this could have affected the reliability of results.
• This study enrolled people having problems conceiving. This is a select population, who may have problems with conception for a number of reasons. The results of this study may not be representative of the population as a whole.
• It is possible that women with high BMIs also have an underlying medical problem, metabolic profile or hormone imbalance, which is responsible for both their obesity and their subfertility. If this is the case, even if the women were to lose weight, this still might not improve their chances of getting pregnant.

There are many reasons why a couple may not be able to conceive, and couples having trouble should consult their doctor who can give advice and refer them for appropriate investigations if needed. Maintaining a healthy weight has benefits for everyone, not just women who want to have a baby.

Sir Muir Gray adds...

This is another good reason for walking an extra 30 minutes a day – the recipe to prevent you gaining weight.