Why are babies getting bigger? Temporal trends in fetal growth and its determinants.

Objectives: To describe temporal trends in fetal "growth" and to examine the roles of sociodemographic, anthropometric, and other determinants.

Study Design: Hospital-based cohort study of 61,437 nonmalformed singleton live births at 22 to 43 weeks' gestational age. Four main measures were examined: (1) birth weight, (2) birth weight-for-gestational-age Z score, (3) small-for-gestational-age (SGA), and (4) large-for-gestational age (LGA), with the latter 3 measures based on a recently developed population-based Canadian reference. Gestational age was based on the last normal menstrual period if confirmed (+/- 1 week) by early ultrasonogram.

Results: The mean birth weight and Z score increased significantly (P <.0001) among infants > or =37 weeks, with a corresponding reduction in % SGA and a rise in % LGA. No consistent trends were seen among births 34 to 36 or < or =33 weeks. When simultaneous changes in maternal prepregnancy body mass index, gestational weight gain, height, cigarette smoking, and other clinical and sociodemographic factors were controlled by using multiple logistic regression, the temporal trends for term infants were no longer evident.

Conclusions: Increases in maternal anthropometry, reduced cigarette smoking, and changes in sociodemographic factors have led to an increase in the weight of infants born at or after term.