Risk factors for small-for-gestational-age infants by customised birthweight centiles: data from an international prospective cohort study

Objective: To identify clinical and ultrasound variables associated with the birth of small-for-gestational-age (SGA) infants by customised centiles, subclassified according to whether their mothers were normotensive or developed hypertensive complications.

Design: Prospective, multicentre cohort study.

Setting: Participating centres of the Screening for Pregnancy Endpoints (SCOPE) study in Auckland, New Zealand, Adelaide, Australia, Manchester and London, UK, and Cork, Ireland.

Population: The 3513 nulliparous participants of the SCOPE study.

Methods: Women were interviewed at 15 ± 1 weeks, and had ultrasound growth measurements and umbilical and uterine Doppler studies at 20 ± 1 weeks. Variables associated with SGA infants were identified using logistic regression.

Main outcome measures: Small for gestational age (i.e. a birthweight of less than the tenth customised centile), normotensive-SGA and hypertensive-SGA. Comparison groups for statistical analyses were non-SGA, normotensive non-SGA and hypertensive non-SGA.

Results: Among 376 (10.7%) SGA infants, 281 (74.7%) were normotensive-SGA and 95 (25.3%) were hypertensive-SGA. Independent risk factors for normotensive-SGA were low maternal birthweight, low fruit intake pre-pregnancy, cigarette smoking, increasing maternal age, daily vigorous exercise, being a tertiary student, head and abdominal circumference of less than the tenth centile and increasing uterine artery Doppler indices at the 20-week scan. Protective factors were: high green leafy vegetable intake pre-pregnancy, and rhesus-negative blood group. Risk factors for hypertensive-SGA were conception by in vitro fertilisation, previous early pregnancy loss and femur length of less than tenth centile at the 20-week scan.

Conclusions: Risk factors for infants who are SGA by customised centiles have been identified in a cohort of healthy nulliparous women. A number of these factors are modifiable; however, further studies are needed to replicate these findings.