Overweight and obese women are at greater risk of giving birth to a preterm baby, new Canadian research finds. And the heavier the woman, the greater the risk.

The findings come from researchers at McMaster University in Hamilton, who pooled data from 84 studies involving more than one million women.

They looked at spontaneous preterm births, in which women suddenly go into labour early, and induced births, in which doctors bring on labour, or perform a C-section.

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. But there was a 30 per cent increased risk of induced preterm birth before 37 weeks among overweight or obese women.

The heavier the woman, the higher the risk of induced preterm birth before 37 weeks, with very obese women at 70 per cent greater risk than normal weight women.

That finding came after the authors accounted for publication bias in the studies, which is the tendency for some studies to be published only if their results are positive.

Overweight or obese women also had a higher risk of very 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 per cent greater risk than normal weight women.

Premature birth is the most common cause of illness and death in babies in the first month of life. The earlier the delivery, the higher the risk the baby will have health problems, such as breathing issues, feeding problems, and other complications.

The findings are relevant to Canada, where 29 per cent of women are overweight, and 23 per cent are obese. Obesity rates are also on the rise, particularly for women aged 25 to 34 -- the age when many women begin having children.

Obesity raises the risk of pre-eclampsia, a condition marked by dangerously high blood pressure that can be life-threatening to mothers and their babies.

Obesity also has been shown to raise the risk of gestational diabetes, a type of diabetes that is diagnosed during pregnancy but that usually resolves afterward. Both conditions raise the risk of early delivery.

Excess weight gain also increases the risk of Caesarean sections, which carries risk for the mother as well.

The authors note that unlike many other causes of preterm birth, maternal obesity is a potentially preventable cause of preterm birth and neonatal death.

"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 optimize their weight before pregnancy," they conclude.