Infants born small for gestational age (SGA) by customised birthweight centiles are at increased risk of adverse outcomes compared with those SGA by population centiles. Risk factors for customised SGA have not previously been described in a general obstetric population.

To determine independent risk factors for customised SGA in a multi‐ethnic New Zealand population.

We performed a retrospective cohort analysis of prospectively recorded maternity data from 2006 to 2009 at National Women’s Health, Auckland, New Zealand. After exclusion of infants with congenital anomalies and missing data, our final study population was 26,254 singleton pregnancies. Multivariable logistic regression analysis adjusted for ethnicity, body mass index, maternal age, parity, smoking status, social deprivation, hypertensive disease, antepartum haemorrhage (APH), diabetes and relevant pre‐existing medical conditions.

Independent risk factors for SGA included obesity (adjusted odds ratio 1.24 [95% CI 1.11–1.39] relative to normal weight), maternal age ≥ 35 years (1.16 [1.05–1.30] relative to 20–29 years), nulliparity (1.13 [1.04–1.24] relative to parity 1), cigarette smoking (2.01 [1.79–2.27]), gestational hypertension (1.46 [1.21–1.75]), pre‐eclampsia (2.94 [2.49–3.48]), chronic hypertension (1.68 [1.34–2.09]), placental abruption (2.57 [1.74–3.78]) and APH of unknown origin (1.71 [1.45–2.00]). Gestational diabetes (0.80 [0.67–0.96]) and type 1 diabetes (0.26 [0.11–0.64]) were associated with reduced risk.

We report independent pregnancy risk factors for customised SGA in a general obstetric population. In contrast to population SGA, obesity is associated with increased risk. Our findings may help identify pregnancies that require increased fetal growth surveillance.