Are Babies Getting Bigger: An Analysis of Birth Weight Trends in New South Wales, 1990–2005

Abstract

During the past decade, increases in babies' birth weight have been reported in Canada and Sweden. Many investigators believe such increases are widespread and are occurring in other countries. The increase has been attributed in part to increases in maternal body mass index, a decrease in smoking during pregnancy, and gestational weight gain. Little confirmatory data for increases in birth weight are available in Australia. This population-based study investigated whether there was an increase in the proportion of babies born large for gestational age (LGA) in New South Wales (NSW) between 1990 and 2005 and identified potential factors associated with any observed increase. Data were collected from the NSW Midwives Data Collection, a surveillance system of all births in NSW. The study population was 1,273,924 live-born singletons delivered at term. Logistic regression analysis was used to investigate the association between LGA, defined as >90th centile for sex and gestational age, and the potential explanatory variables over time. Over the 16-year study period, the proportion of babies born LGA increased among male infants from 9.2% to 10.8% (18% increase) and the percentage of female infants born LGA increased from 9.1% to 11.0% (21.0% increase). The mean birth weight increased significantly during the study period by 23 g among boys and 25 g for girls (P < 0.001). Also, during the study, the percentage of women who smoked during pregnancy decreased from 22.0% to 13.8%, the percentage of women with gestational diabetes increased from 1.1% to 4.6%, and the mean maternal age at delivery increased from 28.4 to 30.4 years. However, only some of the birth weight increase was accounted by the changing trends in smoking, diabetes, or maternal age. These findings indicate that the birth weight of babies born at term is increasing in NSW. Because only a portion of the increase in birth weight can be attributed to decreased smoking, increasing gestational diabetes and increasing maternal age, the determination of other factors responsible for the increase require further investigation.