Table 2. Odds ratios and 95% confidence intervals for preterm birth according to the interpregnancy interval in the 55,203 pregnancies.
| Interpregnancy interval, months | |||||||||||||
|
| |||||||||||||
| <6 | 6–11 | 12–17 | 18–23 | 24–29 | 30–35 | 36–41 | 42–47 | 48–53 | 54–59 | 60–89 | 90–119 | ≥120 | |
| Total population | |||||||||||||
| Number at risk | 1,587 | 5,349 | 9,388 | 8,526 | 6,721 | 4,903 | 3,901 | 2,967 | 2,257 | 1,807 | 4,724 | 1,755 | 1,318 |
| Number of cases | 113 | 238 | 401 | 390 | 269 | 219 | 163 | 143 | 96 | 92 | 253 | 122 | 116 |
| Maternal age-adjusted OR (95% CI) | 1.71 (1.38–2.13) |
1.01 (0.86–1.19) |
0.95 (0.82–1.09) |
1.00 | 0.86 (0.73–1.00) |
0.95 (0.80–1.13) |
0.88 (0.73–1.06) |
1.02 (0.83–1.24) |
0.88 (0.70–1.11) |
1.06 (0.84–1.34) |
1.11 (0.94–1.30) |
1.43 (1.15–1.77) |
1.77 (1.42–2.21) |
| Multivariable OR (95% CI)a | 1.63 (1.30–2.04) |
0.98 (0.83–1.17) |
0.95 (0.82–1.10) |
1.00 | 0.85 (0.72–1.00) |
0.91 (0.77–1.09) |
0.84 (0.69–1.02) |
0.97 (0.79–1.18) |
0.83 (0.66–1.05) |
0.93 (0.73–1.19) |
0.95 (0.80–1.13) |
1.14 (0.91–1.43) |
1.41 (1.11–1.79) |
CI, confidence interval; OR, odds ratio.
aAdjusted for area, maternal age at delivery, body mass index before pregnancy, education levels, smoking during pregnancy, passive smoking during pregnancy, drinking during pregnancy, physical activity during pregnancy, household income during pregnancy, occupation during pregnancy, marital status during pregnancy, number of children, stress during pregnancy, dietary intake of iron during pregnancy, use of anemia medication during pregnancy, intake of dietary folate and folic acid supplementation, the latest pregnancy outcome, spontaneous pregnancy, previous history of preterm birth, hypertensive disorder of pregnancy, and gestational diabetes.