To investigate ethnic variations in risk of preterm birth (PTB), including extreme preterm birth (EPTB) and moderately preterm birth (MPTB), among mothers in an ethnically dense, socially disadvantaged area, and to examine whether any variations were dependent of area deprivation and maternal biological and behavioural factors.

Retrospective cross-sectional study using routinely collected data.

A large UK National Health Service maternity unit.

46 307 women who gave singleton births between April 2007 and March 2016.

PTB defined as <37 weeks of gestation and further classified into EPTB (<28 weeks of gestation) and MPTB (28 to <37 weeks of gestation).

Overall prevalence of PTB was higher (8.3%) compared with the national average (7.8%). Black Caribbean (2.2%) and black African (2.0%) mothers had higher absolute risk of EPTB than white British mothers (1.3%), particularly black Caribbean mothers whose relative risk ratio (RRR) was nearly twice after adjustment for all covariates (RRR=1.93[1.20 to 3.10]). Excess relative risk of EPTB among black African mothers became non-significant after adjustment for prenatal behavioural factors (RRR=1.41[0.99 to 2.01]). Bangladeshi mothers had the lowest absolute risk of EPTB (0.6%), substantially lower than white British mothers (1.3%); the difference in relative risk remained significant after adjustment for area deprivation (RRR=0.59[0.36 to 0.96]), but became non-significant after adjustment for maternal biological factors. Changes were evident in the relative risk of EPTB and MPTB among some ethnic groups compared with the white British on adjustment for different covariates.

Higher than national rates of PTB point to the need for evidence-based antenatal and neonatal care programmes to support preterm babies and their families in ethnically dense socially disadvantaged areas. Differential impact of area deprivation and the role of modifiable behavioural factors highlight the need for targeted preventive interventions for groups at risk.