Table 3. Association Between Birth Volume Category and Severe Maternal Morbidity for Higher-risk and Low-risk Obstetric Patients at Hospitals in Urban Counties.

Annual birth volume	Risk ratio (95% CI)
	Higher-risk patients		Low-risk patients
	Unadjusted	Adjusted	Unadjusted	Adjusted
Low (10-500 births)	0.71 (0.63-0.80)	1.04 (0.93-1.15)	0.84 (0.69-1.03)	0.92 (0.76-1.11)
Medium (501-1000 births)	0.81 (0.69-0.96)	1.06 (0.94-1.19)	0.80 (0.67-0.94)	0.85 (0.73-1.00)
Medium-high (1001-2000 births)	0.91 (0.82-1.00)	1.04 (0.98-1.11)	0.95 (0.83-1.09)	0.99 (0.87-1.14)
High (>2000 births)	1 [Reference]	1 [Reference]	1 [Reference]	1 [Reference]