Objective To determine the socioeconomic gradient of birthweights in England with reference to the prescriptive INTERGROWTH-21^st Birthweight Standard.

Design National cross-sectional study using data from Hospital Episode Statistics.

Setting National Health Service in England.

Participants All singleton babies, live born between 34 weeks’ gestation and 42 weeks’ gestation, between 1 April 2011 and 31 March 2012.

Main outcome measures Birthweight distribution of babies with a birthweight of <10th centile or >90th centile, that is, small for gestational age (SGA) or large for gestational age (LGA) using Index of Multiple Deprivation quintiles as a proxy for socioeconomic status.

Results Of 508 230 babies born alive between 1 April 2011 and 31 March 2012, 38 838 (7.6%) were SGA and 81 026 (15.9%) were LGA. Median birthweight was 3405 g, median z-score was 0.25 (SD 1.06). Birthweight z-score demonstrated a social gradient, from 0.26 (SD 1.1) in the most deprived areas to 0.53 (1.0) in the least deprived. Women in the most deprived areas were twice as likely to have SGA babies using the INTERGROWTH-21^st chart (OR 1.94; 95% CI 1.87 to 2.01) compared with those in the least deprived areas. If all women had the same rate of SGA equivalent to those living in the least deprived areas, approximately 12 410 (30%) fewer babies would be born SGA in England each year.

Conclusions This study gives a measure of the social gradient in singleton SGA and LGA babies across England using an international standard of newborn size at birth.