Table 2.
Risk of preterm birth, early term birth, and small for gestational age by interpregnancy interval in full, unmatched sample (n = 144,733)
| Birth outcome | Interpregnancy interval subgroups n (%) RR (95% CI) aRR (95% CI)a |
|||
|---|---|---|---|---|
| < 6 months n = 16,903 | 6–11 months n = 40,985 | 12–17 months n = 48,710 | 18–23 months n = 38,135 | |
| < 32 weeks GA | 173 (1.0) | 225 (0.6) | 218 (0.5) | 145 (0.4) |
| 3.03 (2.43, 3.78) | 1.50 (1.22, 1.85) | 1.19 (0.96, 1.46) | Ref | |
| 2.38 (1.88, 3.00) | 1.37 (1.11, 1.69) | 1.19 (0.96, 1.46) | Ref | |
| 32–36 weeks GA | 1292 (7.6) | 2111 (5.2) | 2109 (4.3) | 1632 (4.3) |
| 1.92 (1.78, 2.06) | 1.24 (1.16, 1.32) | 1.02 (0.96, 1.09) | Ref | |
| 1.67 (1.54, 1.80) | 1.18 (1.11, 1.26) | 1.02 (0.95, 1.09) | Ref | |
| < 37 weeks GA (any preterm) | 1465 (8.7) | 2336 (5.7) | 2327 (4.8) | 1777 (4.7) |
| 1.98 (1.85, 2.12) | 1.26 (1.18, 1.34) | 1.03 (0.97, 1.10) | Ref | |
| 1.70 (1.58, 1.83) | 1.19 (1.12, 1.27) | 1.03 (0.97, 1.10) | Ref | |
| 37–38 weeks GA (early term) | 5106 (30.2) | 11,349 (27.7) | 12,750 (26.2) | 9810 (25.7) |
| 1.23 (1.19, 1.27) | 1.09 (1.06, 1.12) | 1.01 (0.99, 1.05) | Ref | |
| 1.17 (1.13, 1.21) | 1.07 (1.04, 1.10) | 1.01 (0.99, 1.05) | Ref | |
| SGA | 1234 (7.3) | 2494 (6.1) | 2735 (5.6) | 2226 (5.8) |
| 1.25 (1.17, 1.34) | 1.04 (0.98, 1.10) | 0.96 (0.91, 1.02) | Ref | |
| 1.10 (1.02, 1.19) | 1.00 (0.95, 1.06) | 0.97 (0.91, 1.02) | Ref | |
aRR adjusted relative risk, CI confidence interval, GA gestational age at birth, RR relative risk, SGA small for gestational age
aRisks adjusted for maternal characteristics in Table 1