Table 2. Associations Between Annual Birth Volume Category and Severe Maternal Morbidity (SMM) for Hospitals in Urban and Rural US Counties, Only Including Urban Hospitals With 600 or Fewer Annual Births.
| Births, No. | Total patients, No. | SMM incidence, No. (%) | Risk ratio (95% CI) | |
|---|---|---|---|---|
| Unadjusted | Adjusted | |||
| Low-volume hospitals in urban counties | ||||
| 10-110 | 5308 | 37 (0.70) | 1.24 (0.88-1.76) | 1.28 (0.93-1.77) |
| 111-240 | 24 238 | 98 (0.40) | 0.72 (0.54-0.95) | 0.90 (0.72-1.12) |
| 241-460 | 178 076 | 875 (0.49) | 0.87 (0.71-1.08) | 0.96 (0.83-1.12) |
| 461-600 | 176 027 | 989 (0.56) | 1 [Reference] | 1 [Reference] |
| Rural counties | ||||
| 10-110 | 8182 | 57 (0.70) | 1.48 (1.01-2.18) | 1.65 (1.14-2.39) |
| 111-240 | 59 374 | 324 (0.55) | 1.16 (0.90-1.49) | 1.37 (1.10-1.70) |
| 241-460 | 175 176 | 967 (0.55) | 1.17 (0.96-1.44) | 1.26 (1.05-1.51) |
| >460 | 277 221 | 1304 (0.47) | 1 [Reference] | 1 [Reference] |