Background: Despite evidence supporting the safety of vaginal birth after caesarean section (VBAC), rates are low in many countries. Methods: OptiBIRTH investigated the effects of a woman-centred intervention designed to increase VBAC rates through an unblinded cluster randomised trial in 15 maternity units with VBAC rates < 35% in Germany, Ireland and Italy. Sites were matched in pairs or triplets based on annual birth numbers and VBAC rate, and randomised, 1:1 or 2:1, intervention versus control, following trial registration. The intervention involved evidence-based education of clinicians and women with one previous caesarean section (CS), appointment of opinion leaders, audit/peer review, and joint discussions by women and clinicians. Control sites provided usual care. Primary outcome was annual hospital-level VBAC rates before the trial (2012) versus final year of the trial (2016). Between April 2014 and October 2015, 2002 women were recruited (intervention 1195, control 807), with mode-of-birth data available for 1940 women. Results: The OptiBIRTH intervention was feasible and safe across hospital settings in three countries. There was no statistically significant difference in the change in the proportion of women having a VBAC between intervention sites (25.6% in 2012 to 25.1% in 2016) and control sites (18.3 to 22.3%) (odds ratio adjusted for differences between intervention and control groups (2012) and for homogeneity in VBAC rates at sites in the countries: 0.87, 95% CI: 0.67, 1.14, p = 0.32 based on 5674 women (2012) and 5284 (2016) with outcome data. Among recruited women with birth data, 4/1147 perinatal deaths > 24 weeks gestation occurred in the intervention group (0.34%) and 4/782 in the control group (0.51%), and two uterine ruptures (one per group), a rate of 1:1000.(Continued on next page)