Objective Small-for-gestational age (SGA) birthweight is associated with adverse pregnancy outcome. The authors wanted to examine the associated factors and relative contributions they make to the babies born SGA in a general maternity population.

Method Data from 23 768 pregnancies delivered between July and December 2009 collected on new regional electronic database. The cohort consisted of 21 077 (89%) cases with complete data. Outcome (SGA) was defined as birthweight below the 10th customised centile. Variables for the logistic regression analysis included maternal characteristics such as age, height, weight, parity and ethnicity, smoking and the worst quintiles of the area based ‘index of multiple deprivation’ (IMD Q4 & Q5). Results were expressed as relative risk (RR) and population attributable risk (PAR).

Results The SGA birthweight rate in this cohort was 14.4%. Both low and high BMI increased the risk of SGA, as did maternal age ≥35, while age <20 was protective. Both high quintiles of deprivation were associated with SGA, and together contributed 10.9% of PAR. However the strongest risk was associated with smoking, with RR of 2.1 and PAR 17.8.

Conclusion Smoking and social deprivation are separate factors contributing to high SGA rates in our population. While the effect of smoking on fetal growth is well known, further work is needed to elucidate whether the higher rate of SGA babies born in areas of deprivation are due to poor maternal nutrition or other factors.