Table 3. Adjusted odds ratios (aOR) and 95% confidence intervals (CI) between interpregnancy interval and adverse birth outcomes among births in between-women* and within-women** analyses across the four countries.
| Outcome by country | Interpregnancy interval | ||||||
|---|---|---|---|---|---|---|---|
| <6 months | 6–11 months | 12–17 months | 18–23 months | 24–59 months | 60–119 months | ≥120 months | |
| PTB | aOR (95% CI) | ||||||
| Australia | |||||||
| Between-women | 1.68 (1.63, 1.73) | 1.13 (1.10, 1.16) | 0.99 (0.97, 1.02) | Ref | 1.16 (1.13, 1.19) | 1.59 (1.55, 1.64) | 2.14 (2.03, 2.27) |
| Within-women | 1.22 (1.15, 1.29) | 1.08 (1.03, 1.14) | 1.00 (0.95, 1.05) | Ref | 1.06 (1.02, 1.11) | 1.35 (1.27, 1.43) | 1.71 (1.52, 1.92) |
| Finland | |||||||
| Between-women | 1.62 (1.53, 1.70) | 1.06 (1.01, 1.10) | 0.99 (0.95, 1.10) | Ref | 1.12 (1.08, 1.16) | 1.41 (1.35, 1.47) | 1.73 (1.61, 1.86) |
| Within-women | 1.00 (0.91, 1.09) | 1.02 (0.95, 1.10) | 0.95 (0.88, 1.02) | Ref | 1.00 (0.94, 1.07) | 1.26 (1.16, 1.37 | 1.67 (1.44, 1.95) |
| Norway | |||||||
| Between-women | 2.06 (1.96, 2.17) | 1.17 (1.12, 1.22) | 1.02 (0.98, 1.06) | Ref | 1.11 (1.08, 1.15) | 1.51 (1.45, 1.57) | 1.91 (1.82, 2.02) |
| Within-women | 0.97 (0.88, 1.06) | 1.01 (0.93, 1.09) | 1.03 (0.96, 1.12) | Ref | 1.08 (1.02, 1.15) | 1.38 (1.29, 1.49) | 1.71 (1.51, 1.93) |
| California | |||||||
| Between-women | 1.42 (1.39, 1.45) | 1.16 (1.14, 1.18) | 1.06 (1.05, 1.08) | Ref | 1.12 (1.10, 1.14) | 1.41 (1.39, 1.44) | 1.82 (1.76, 1.88) |
| Within-women | 1.13 (1.09, 1.18) | 1.10 (1.06,1.14) | 1.07 (1.03,1.11) | Ref | 1.00(0.97, 1.03) | 1.17 (1.13, 1.22) | 1.42 (1.31, 1.54) |
| Spontaneous PTB | |||||||
| Australia | |||||||
| Between-women | 1.93 (1.85, 2.01) | 1.22 (1.18, 1.27) | 1.03 (0.99, 1.07) | Ref | 1.13 (1.10, 1.17) | 1.56 (1.50, 1.63) | 1.99 (1.84, 2.16) |
| Within-women | 1.66 (1.54, 1.79) | 1.30 (1.22, 1.39) | 1.07 (1.00, 1.14) | Ref | 0.96 (0.90, 1.02) | 1.10 (1.02, 1.19) | 1.28 (1.09, 1.51) |
| Finland | |||||||
| Between-women | 1.77 (1.66, 1.88) | 1.14 (1.09, 1.20) | 1.04 (0.99, 1.09) | Ref | 1.15 (1.10, 1.20) | 1.45 (1.37, 1.52) | 1.76 (1.62, 1.92) |
| Within-women | 1.32 (1.19, 1.46) | 1.22 (1.12, 1.34) | 1.04 (0.96, 1.14) | Ref | 1.05 (0.96, 1.13) | 1.35 (1.22, 1.49) | 1.77 (1.47, 2.13) |
| Norway | |||||||
| Between-women | 2.04 (1.92, 2.18) | 1.22 (1.16, 1.28) | 1.04 (0.99, 1.09) | Ref | 1.06 (1.02, 1.10) | 1.40 (1.34, 1.47) | 1.78 (1.64, 1.90) |
| Within-women | 1.21 (1.08, 1.36) | 1.10 (1.00, 1.21) | 1.04 (0.95, 1.14) | Ref | 0.93 (0.86, 1.01) | 1.12 (1.02, 1.23) | 1.29 (1.09, 1.51) |
| California | |||||||
| Between-women | 1.67 (1.62, 1.72) | 1.15 (1.12, 1.18) | 1.03 (1.00, 1.06) | Ref | 1.15 (1.12, 1.17) | 1.52 (1.48, 1.56) | 1.96 (1.88, 2.05) |
| Within-women | 1.34 (1.26, 1.42) | 1.12 (1.06, 1.18) | 1.06 (1.00, 1.13) | Ref | 1.05 (1.00, 1.10) | 1.35 (1.27, 1.43) | 1.65 (1.47, 1.86) |
| SGA | |||||||
| Australia | |||||||
| Between-women | 1.10 (1.07, 1.13) | 0.99 (0.97, 1.01) | 0.98 (0.96,1.01) | Ref | 1.18 (1.15, 1.20) | 1.65 (1.61, 1.69) | 2.08 (1.98, 2.19) |
| Within-women | 0.94 (0.87, 0.99) | 0.99 (0.95, 1.04) | 1.03 (0.99,1.08) | Ref | 1.04 (1.00, 1.09) | 1.24 (1.17, 1.31) | 1.58 (1.40, 1.78) |
| Finland | |||||||
| Between-women | 1.07 (1.03, 1.12) | 0.95 (0.92, 0.98) | 0.98 (0.95, 1.01) | Ref | 1.09 (1.06, 1.12) | 1.35 (1.31, 1.40) | 1.72 (1.63, 1.81) |
| Within-women | 0.89 (0.82, 0.96) | 0.99 (93, 1.05) | 1.00 (0.95, 1.07) | Ref | 1.08 (1.02, 1.13) | 1.24 (1.16, 1.32) | 1.80 (1.59, 2.03) |
| Norway | |||||||
| Between-women | 1.28 (1.22, 1.35) | 1.08 (1.05, 1.12) | 1.02 (0.99, 1.05) | Ref | 1.11 (1.08, 1.14) | 1.42 (1.37, 1.46) | 1.83 (1.75, 1.92) |
| Within-women | 0.88 (0.81, 0.97) | 0.98 (0.91, 1.05) | 0.97 (0.91, 1.03) | Ref | 1.05 (1.00, 1.11) | 1.26 (1.18, 1.34) | 1.63 (1.45, 1.82) |
| California | |||||||
| Between-women | 1.42 (1.39, 1.46) | 1.08 (1.06, 1.10) | 1.00 (0.98, 1.02) | Ref | 1.13 (1.11, 1.15) | 1.37 (1.34, 1.40) | 1.64 (1.59, 1.70) |
| Within-women | 1.26 (1.20, 1.31) | 1.05 (1.01, 1.09) | 1.00 (0.96, 1.04) | Ref | 1.03 (1.00, 1.07) | 1.17 (1.12, 1.22) | 1.52 (1.39, 1.67) |
aOR—adjusted odds ratio. CI—confidence interval. IPI—interpregnancy interval. PTB—preterm birth. SGA—small for gestational age.
*Odds ratios calculated using between-women analyses for women with ≥2 births/ ≥1 IPI after prognostic score adjustment for maternal age, parity, and year of birth.
**Odds ratios calculated using within-women analyses for women with ≥3 births/ ≥2 IPIs after prognostic score adjustment for maternal age, parity, and year of birth