An observational study of 20,499 consecutive live births 2008-2021 at one hospital in Auckland, Aotearoa New Zealand. Index birth was term, cephalic, singleton. Multivariable analysis was used.

Outcome measures SpPTB <25 <28 <32 and <37 weeks

Key findings

•Operative vaginal birth was not associated with subsequent spPTB

•CS in labour at cervical dilatations ≥5cm is independently associated with increased risk of subsequent spPTB

•Full cervical dilatation CS is associated with a RR 7.7 (3.2-18.8) of spPTB at <25 weeks; RR 7.7 (3.9-15.3) at <28 weeks; RR 4.5 (2.6-7.8) at <32 weeks; RR 2.8 (1.8-4.2) at <37 weeks

•Uterine incision extension into the vagina (14 cases) is associated with a RR 5.1 (1.8-14.5) of subsequent spPTB

•Cervical dilatation is associated with increased spPTB independent of uterine incision extension

•Using a manoeuvre* to assist delivery of the fetal head is associated with reduced risk of subsequent spPTB: RR 0.4 (0.2-0.9)

Interpretation

•CS in labour ≥ 5cm cervical dilatation is associated with subsequent spontaneous preterm birth

•Subsequent spontaneous preterm birth is increased at all gestation thresholds <25, <28, <32, and <37weeks

•Uterine incision extension into the vagina is associated with an increased risk independent of cervical dilatation