Given a pregnancy’s birth order and year, the probability of
different combinations of prior cesarean and vaginal births
was calculated to determine the number of previous cesareans
a woman had experienced and the probability that the present
birth would be delivered via cesarean. We relied on data for
the rates of primary cesarean and VBAC in the years leading
up to the model year to make these calculations. This resulted
in a range of zero to three prior cesareans (Figure 1). Given
the number of prior cesareans for each pregnancy, the
probabilities of placenta previa, placenta accreta with or
without previa, trial of labor, success of a trial of labor,
cesarean hysterectomy, need for blood transfusion, and
maternal mortality varied.