Why are babies in Canada getting smaller?

Introduction Recent evidence from the United States and Canada suggests an unexplained increase in small-for-gestational-age (SGA) births (<10th percentile). This study aimed to identify reasons for the recent increase in SGA births in Canada. Data and methods Using Canada's Vital Statistics - Birth Database, the study population included all singleton live births, 2000 to 2016, inclusive. Temporal changes in birth weight (grams), birth weight for gestational age z-scores, and SGA births were examined. Multivariable logistic regression was used to determine if the increased risk of an SGA birth over time was eliminated or attenuated by adjusting for selected individual and sociodemographic factors that have previously been associated with SGA births. Results There were 5,941,820 singleton live births in Canada between 2000 and 2016. Mean birth weight for all births decreased from 3,442 grams in 2000, to 3,367 grams in 2016, while SGA birth increased from 7.2% in 2000 to 8.0% in 2016. The multivariable model showed higher odds of SGA birth among births to parents born outside of Canada, unmarried women, older women, nulliparous women and women residing in low income neighborhoods. After adjusting for sociodemographic factors, the crude 12% increase in odds of SGA birth in 2016 compared to 2000 (95% Confidence Interval (CI): [10 to 14%]) was attenuated, but not eliminated (adjusted odds ratio for calendar time 1.08 (95% CI: [1.06, 1.10])). Interpretation This study identified a decrease in fetal size in Canada between 2000 and 2016. The rise in SGA births in Canada was explained only partly as a result of concurrent changes in the demography of childbirth. [ABSTRACT FROM AUTHOR]