A research team led by Gordon Smith examined more than 500,000 entries collected by the Scottish Morbidity Record between 1980 and 2005 to determine the association between maternal age and the outcome of labor.
The proportion of women ages 35 to 39 entering their first pregnancy increased seven-fold, while there was a 10-fold increase for women age 40 or older. At the same time, the Cesarean rate more than doubled.
The researchers estimate that 38 percent of these additional procedures would have been avoided if the maternal age distribution had remained static.
Hypothesizing there was a biological cause, the researchers found advancing age was associated with impaired uterine function, in particular a reduced degree of spontaneous contraction and change in the type of contraction. The reduction is thought to result in poor progression of labor and thus the increased rate of Cesarean section.
The findings are published in Public Library of Science Medicine.