Background: Regionalisation programmes aim to ensure that very preterm infants are born in level III units (inborn) through antenatal referral or transfer. Despite widespread knowledge about better survival without disability for inborn babies, 10%-30% of women deliver outside these units (outborn). Objective: To investigate risk factors associated with outborn deliveries and to estimate the proportion that were probably or possibly avoidable. Methods: We used a national French population-based cohort including 2205 women who delivered between 24 and 30+6 weeks in 2011. We examined risk factors for outborn delivery related to medical complications, antenatal care, sociodemographic characteristics and living far from a level III unit using multivariable binomial regression. Avoidable outborn deliveries were defined by pregnancy risk (obstetric history, antenatal hospitalisation) and time available for transfer. Results: 25.0% of women were initially booked in level III, 9.1% were referred, 49.8% were transferred, and 16.1% had outborn delivery. Risk factors for outborn delivery were gestational age <26 weeks (adjusted relative risk (aRR) 1.37, 95% confidence interval (CI) 1.13, 1.66), inadequate antenatal care (aRR 1.39, 95% CI 1.10, 1.81), placental abruption (aRR 1.66, 95% CI 1.27, 2.17), and increased distance to the closest level III unit ((aRR 2.79, 95% CI 2.00, 3.92) in the 4th versus 1st distance quartile). Among outborn deliveries, 16.7% were probably avoidable, and 25.6% possibly avoidable, which could increase the proportion of inborn deliveries between 85.9% and 92.9%. Avoidable outborn deliveries were mainly associated with gestational age, intrauterine growth restriction, preterm premature rupture of membranes, and haemorrhage, but not distance. Conclusions: Our study identified some modifiable risk factors for outborn delivery; however, when regionalised care relies heavily on antenatal transfer, as it does in France, only some outborn deliveries may be prevented. Earlier referral of high-risk women will be needed to achieve full access to tertiary care.