NEW ORLEANS – Weight gain between pregnancies decreases the likelihood of a successful vaginal birth after cesarean section, according to findings from a large population-based retrospective cohort study.
The more weight a woman gains between pregnancies, the lower her chances are of a successful vaginal birth after cesarean section (VBAC); conversely, overweight and obese women who lose weight between pregnancies may improve their chances of a successful VBAC, Dr. Lisa S. Callegari reported at the annual meeting of the American College of Obstetricians and Gynecologists.
The overall VBAC success rate among 8,861 women included in the study was 66%, but differences were seen with VBAC success based on prepregnancy body mass index category: The success rate was 70% for normal-weight women, 62% for overweight women, and 56% for obese women, said Dr. Callegari of the University of Washington, Seattle.
After adjustment for a number of factors, including demographics, smoking status, interpregnancy interval, birth year of second pregnancy (to account for temporal changes in VBAC practices), and prenatal care adequacy, weight loss among normal-weight women was not found to be associated with VBAC success, but weight gain of between one and two BMI units (about 6-12 pounds) among normal-weight women was associated with a 7% decrease in the VBAC success rate, compared with weight maintenance.
High weight gain, defined as an increase of more than two BMI units (about 12 pounds) between pregnancies, was associated with a 13% decrease in the VBAC success rate, compared with weight maintenance.
For overweight and obese women, moderate and high weight gain did not affect VBAC success, but weight loss in overweight women was associated with a 12% increase in VBAC success, and weight loss equivalent to 1 or more BMI units in obese women was associated with a 24% increase in VBAC success, Dr. Callegari said.
The findings are based on an analysis of longitudinal birth certificate data linked with hospitalization records between 1992 and 2009. Women who were included underwent primary cesarean delivery for their first birth, and attempted a trial of labor for their second birth.
Although limited by the use of birth certificate data, and also by a large number of cases with missing BMI information, the findings are bolstered by the use of population-based data, an observed dose-response effect, and the biologic plausibility for the findings, Dr. Callegari said.
The findings are important because increasing the rates of VBAC has been proposed as a strategy for decreasing cesarean section rates. However, VBAC rates have been declining in recent years, falling from 23% in 1996 to less than 10% currently. The causes are multifactorial, but these findings suggest that the modifiable risk factors of overweight and obesity are among them, Dr. Callegari said.
"In terms of clinical implications, normal-weight women should return to within one BMI unit of their prepregnancy weight, and overweight and obese women should lose greater than or equal to one BMI unit between pregnancies in order to optimize their chances of VBAC success," she concluded.
Dr. Callegari reported having no disclosures.