To determine whether the proportion of babies born large for gestational age (LGA) in New South Wales has increased, and to identify possible reasons for any increase.

Population-based study using data obtained from the NSW Midwives Data Collection, a legislated surveillance system of all births in NSW.

All 1 273 924 live-born singletons delivered at term (≥ 37 complete weeks’ gestation) in NSW from 1990 to 2005.

LGA, defined as > 90th centile for sex and gestational age using 1991–1994 Australian centile charts; maternal factors associated with LGA were assessed using logistic regression.

The proportion of babies born LGA increased from 9.2% to 10.8% (18% increase) for male infants and from 9.1% to 11.0% (21% increase) for female infants. The mean birthweight increased by 23 g for boys and 25 g for girls over the study period. Increasing maternal age, higher rates of gestational diabetes and a decline in smoking contributed significantly to these increases, but did not fully explain them.

There is an increasing trend in the proportion of babies born LGA, which is only partly attributable to decreasing maternal smoking, increasing maternal age and increasing gestational diabetes.