Data source
We obtained data from the Dutch Perinatal Registry (Perined, formerly
known as Perinatale Registratie Nederland [PRN] ). The
Registry contains information on pregnancy, birth, and maternal and
perinatal characteristics and outcomes (Appendix S1) for 97% of all
births in the Netherlands (27-29). All characteristics
except birth asphyxia, registration of which started only in 2008, were
recorded routinely during the entire study period. Perined combines
births in primary and hospital care by using a validated linkage method(30, 31).
The Perined dataset did not contain information on the number of
previous caesarean sections in women with a parity of two or more. This
hampered assessment of vaginal births after one previous caesarean
section in addition to one or more vaginal births. Therefore, in order
to ensure a homogeneous group of participants we included only para 1
women with one previous caesarean section (n= 144,166; 67.8% of all
women in Robson group 5, meaning all women with one or more previous
caesarean sections in addition to any number of vaginal births). We
excluded women from analysis in whom mode of birth in the current
pregnancy was unknown (n=858; 0.6% of Robson group 5 with a parity of
one).