Preterm births higher among overweight and obese mothers

Overweight and obese women are at greater risk of giving birth to a preterm baby compared with normal weight women, finds a study published in the British Medical Journal today.

The authors believe that overweight and obese women should have counselling before pregnancy so that they are aware of these risks and can try to modify their weight before pregnancy. They also stress the need for appropriate surveillance by health professionals during pregnancy.

Overweight and obesity is now the most common pregnancy complication in many developed countries and also some developing countries. For example, in the United Kingdom, 33% of pregnant women are overweight or obese. In India, 26% of pregnant women are overweight and a further 8% are obese, while in China, 16% are overweight or obese.

Preterm birth and low birth weight are the leading causes of infant death and illness throughout childhood. However, there is still uncertainty about the impact of a mother's weight on both preterm birth and low birth weight.

So a team of researchers in Canada analysed the results of 84 studies to assess the effect of maternal weight on preterm birth (before 37 weeks) and low birth weight (below 2500g) in singleton pregnancies in both developed and developing countries.

They found that the overall risk of preterm birth before 37 weeks was not significantly different among overweight or obese women compared with normal weight women.

However, there was a 30% increased risk of induced preterm birth before 37 weeks among overweight or obese women after accounting for publication bias, which is the tendency for studies to be published only if their results are positive. The heavier the woman, the higher the risk of induced preterm birth before 37 weeks, with very obese women at 70% greater risk than normal weight women.

Overweight or obese women also had a higher risk of early preterm birth (before 32 or 33 weeks). Again, the heavier the woman, the higher the risk of early preterm birth, with very obese women at 82% greater risk than normal weight women.

Although overweight or obese women had a lower risk of delivering a low birth weight baby than normal weight women, especially in developing countries, this effect disappeared after publication bias was taken into account. "Clinicians need to be aware that maternal overweight or obesity is not protective against low birth weight and consider surveillance when indicated," warn the authors.

"Ideally, overweight or obese women should have pregnancy counselling so that they are informed of their perinatal risks and can try to optimise their weight before pregnancy," they conclude.