Study population
We conducted a retrospective cohort study of non-anomalous, singleton fetuses receiving an outpatient obstetrical ultrasound between 28 weeks, 0 days and 41 weeks, 6 days gestational age at the British Columbia (BC) Women’s Hospital, Vancouver, British Columbia, Canada between April 1 2000 to March 31 2011. BC Women’s Hospital is a tertiary care teaching hospital of the University of British Columbia. It is the primary fetal and children’s referral centre for the province of British Columbia, with an annual delivery volume of approximately 7000 births. Ultrasound records were linked with British Columbia population-level health and education databases, including the BC Perinatal Data Registry, which contains abstracted obstetrical and neonatal medical records for >99% of births in the province, and results from routine province-wide kindergarten readiness testing. We restricted our analytic cohort to fetuses with a valid estimate of fetal weight at or beyond 28 weeks’ gestation, with no major anomalies (based on the US Center for Disease Control Definition of major anomalies). We obtained obstetrical and neonatal health outcomes for all births in British Columbia to help describe the source population from which our ultrasound cohort was drawn.