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Severe obesity doubles risk of preterm delivery, study shows

BMJ 2012; 345 doi: (Published 31 October 2012) Cite this as: BMJ 2012;345:e7338
  1. Anders Hansen
  1. 1Stockholm

Being obese increases the risk of preterm birth, with the heaviest women (body mass index of 40 or more) nearly twice as likely to deliver before 37 weeks of gestation as women whose weight was normal, a study has found.

For the study, researchers from Sahlgrenska University Hospital in Gothenburg used data from the Norwegian mother and child cohort study to look at 83 544 pregnancies that occurred between 1999 and 2008.1 Information on weight and height before pregnancy was reported by the women themselves.

The study, published in the American Journal of Obstetrics and Gynecology, found that overall 5.1% of all women had a preterm delivery, defined as delivery before 37 weeks’ gestation. Of the women whose weight was normal (BMI between 18.5 and 24.9) 4.7% delivered preterm.

The highest rate of preterm delivery was 9.2%, in women with grade III obesity (BMI≥40). The next group was women who were underweight (BMI<18.5), of whom 8.1% delivered early, then women with grade II obesity (BMI 35 to 39.9), who had a preterm delivery rate of 7.3%, and women with grade I obesity (BMI 30 to 34.9), whose rate was 7.1%

The researchers divided preterm delivery into two groups: early preterm delivery for births between 22 and 31 weeks and 6 days’ gestation and late preterm delivery for births between 32 and 36 weeks and 6 days’ gestation. Women with grade III obesity were found to be 3.5 times (95% confidence interval 1.21 to 8.99 times) as likely to have an early preterm delivery as women of normal weight.

This is not the first time that overweight and obesity have been linked to an increased risk of preterm delivery. However, several previous studies indicated that obesity increases the risk of hypertension and diabetes, which in turn increase the risk of preterm delivery. The current study shows that obesity in itself seems to be a risk factor regardless of diabetes or hypertension.

Preterm delivery is the leading cause of neonatal morbidity and causes three million deaths each year worldwide.2 The World Health Organization estimated that 15 million children were born prematurely each year, 10% of the total.3 At the same time maternal obesity is a growing global problem.

The authors concluded that weight reduction before pregnancy could help overweight women reduce the risk of a preterm delivery.


Cite this as: BMJ 2012;345:e7338


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