Lean women have lower risk of caesarean

The body mass index (BMI) before pregnancy can also be used to predict the risk of caesarean section in women who have full-term deliveries. Previous studies have found that obese women have an increased risk of caesarean section. Researchers from the Groupe Hospitalier Sud-Reunion, Saint-Pierre, La Reunion, France found that there is a linear association between all degrees of maternal corpulence before being pregnant and the risk of caesarean section. Many people, including obstetricians are under the impression that the relationship is U-shaped, with women with a normal BMI having the lowest rate of caesarean sections, obese women having the highest and very lean women also having a higher rate of caesarean. The researchers used BMI, calculated as 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 - anything higher or lower is underweight or overweight, respectively. Those with a BMI of more than 30 are considered obese. The researchers examined the data for more than 17,000 single live births that had taken place at the island hospital over a four and a half 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 caesarean section was found. The leanest mothers had the highest rates of vaginal delivery, and the most corpulent had the lowest. The caesarean 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. The possible contributing factor could be that, on average, leaner mothers have lighter babies, and the fatty tissue is poorly compressible leading to a tapering of the pelvis. The researchers concluded that obese women tend to have heavier babies and a relatively narrower pelvis and therefore have a higher caesarean rate.