There is a linear association between pre-pregnancy body mass index (BMI) and the risk of cesarean section in women who have full-term deliveries, French researchers report in the current issue of BJOG International.

It is well known that women who are obese have an increased risk of cesarean section, Dr. Pierre-Yves Robillard told Reuters Health. “What we show in this study is that there is a linear association between all degrees of maternal corpulence before being pregnant and the risk of cesarean section.”

Robillard’s team used BMI, the ratio of height to weight, to determine if a mother was overweight/obese, underweight or in the normal range. The BMI range for normal weight is between 18.8 and 25.0 - anything higher or lower is underweight or overweight, respectively. Those with a BMI of more than 30.0 are considered obese.

Many people, including obstetricians, Robillard added, are under the impression that the relationship is U-shaped, with women with a “normal” BMI having the lowest rate of cesarean sections, obese women having the highest and very lean women also having a higher rate of cesarean. However, this appears not to be the case.

Robillard of Groupe Hospitalier Sud-Reunion, Saint-Pierre, La Reunion, and colleagues, including lead author Dr. Georges Barau, examined data for more than 17,000 single live births that had taken place at the island hospital over a 4.5-year period.

When pre-pregnancy BMIs, ranging from 10 to 45, were determined for almost all (97.1 percent) of the mothers, a significant linear association with the risk of cesarean section was found. The leanest mothers had the highest rates of vaginal delivery, and the most corpulent had the lowest.

The cesarean rate was 9.3 percent in those with a BMI less than 15 and rose steadily with BMI, reaching about 30 percent in those with a BMI of 40 or more.

Among possible contributing factors, Robillard observed, is that, on average, leaner mothers have lighter babies, and that fatty tissue is poorly compressible leading to a tapering of the pelvis.

The investigators conclude that obese women tend to have heavier babies and a relatively narrower pelvis and therefore have a higher cesarean rate.

SOURCE: BJOG International, October 2006.