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Relative risk (RR) of undergoing obstetric ultrasonography in fiscal year 2006/07 relative to fiscal year 1996/97. Data were adjusted for maternal age, income quintile, rural versus urban place of residence, any diabetes mellitus, any hypertension, any other maternal comorbidity, genetics consultation and amniocentesis — all in the index pregnancy — and a history of complications in a prior pregnancy. For all within-stratum interaction terms, p < 0.001. CI = confidence interval.
