Women who are obese during pregnancy may be more likely to have children with birth defects such as spina bifida, heart problems and cleft palate, researchers say.

The study in Wednesday's Journal of the American Medical Association reviewed 18 studies that looked at the relationship between a woman's weight and her baby's health.

Women who were obese at the start of their pregnancy had a slightly increased risk for having babies with spina bifida, heart defects, cleft lip or cleft palate.

The risk for experiencing a neural tube defect, which are caused by the incomplete development of the brain or spinal cord, was .47 per 1,000 births greater among obese women than those with a healthy body mass index, Katherine Stothard and colleagues from Britain's Newcastle University found.

For one such defect, spina bifida, the risk was more than double.

For a heart anomaly, the risk was .61 per 1,000 births greater.

While the risk was small, experts said more attention should be paid to a woman's weight given the growing obesity epidemic.

Since birth defects affect only about two to four per cent of pregnancies, the absolute risk for obese women is low, researchers said.

The study's authors proposed several ways that obesity might contribute to birth defects:

  • Undiagnosed diabetes leads to the defects.
  • Nutritional deficiencies in obese pregnant women, such as a lack of folic acid.
  • Ultrasound is more difficult to perform on heavy women, which means birth defects may be less likely to be detected early in pregnancy.

The study's authors suggested that women who are thinking about trying for a baby should check their own weight first and eat a healthy diet.

Obesity in pregnancy is linked to gestational diabetes, high blood pressure, an increased risk of blood clots, a higher risk of infection and an increased risk of caesarian delivery.

Babies born to obese mothers are at risk of being too large and have an increased risk of perinatal death.

In general, obesity increases the risk of diseases such as Type 2 diabetes and heart problems.