The objective of this study was to determine whether, and to what degree, the change in the vaginal birth after caesarean section (VBAC) rate is due to a change in the characteristics of the obstetric population, the undertaking of a trial of labour (TOL), or the tendency to abandon a TOL once it has been initiated. All women with one prior low transverse caesarean section (CS) and a vertex singleton gestation at term were identified in a registry of CS deliveries occurring at eight academic centres during a 4-year period (1999-2002). Women were classified by their predicted chance of VBAC and year-to-year differences were analysed. Of the 9643 women who met criteria for analysis, 5334 (55.3%) underwent a TOL. From 1999 to 2002, the VBAC rate underwent a steady decline: 51.8% to 45.1% to 37.4% to 29.8% (P < 0.001). Although there were some changes in the characteristics of the population that predispose to successful VBAC, as well as some reduction in the chance that a VBAC is successful once a TOL is undertaken, the most pervasive reason for this decline was that women became increasingly likely to forego a TOL, regardless of their likelihood of vaginal delivery. Based on these results, it appears that the change over time in the VBAC rate is multifactorial, although the greatest change has been a decrease in the frequency with which women undertake a TOL, and this change is observed in all categories of the chance of a successful TOL.
© 2010 Blackwell Publishing Ltd.