Table 1. Mean and prevalence of characteristics according to interpregnancy interval in the 55,203 pregnancies.
| Interpregnancy interval (months) | P for trend | |||||||||||||
|
| ||||||||||||||
| <6 | 6–11 | 12–17 | 18–23 | 24–29 | 30–35 | 36–41 | 42–47 | 48–53 | 54–59 | 60–89 | 90–119 | ≥120 | ||
| Participants, n | 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 | |
| Age at delivery, year | 28.5 | 29.7 | 30.7 | 31.4 | 32.1 | 32.5 | 32.9 | 33.1 | 33.6 | 33.8 | 34.3 | 35.3 | 37.0 | <0.001 |
| Age at first birth, year | 25.2 | 26.7 | 27.4 | 27.7 | 27.9 | 27.8 | 27.7 | 27.3 | 27.3 | 26.9 | 26.1 | 24.6 | 23.1 | <0.001 |
| BMI before pregnancy, kg/m2 | 22.2 | 21.4 | 21.1 | 21.1 | 21.2 | 21.3 | 21.5 | 21.5 | 21.4 | 21.6 | 21.7 | 22.0 | 22.1 | <0.001 |
| College or higher education, % | 9.7 | 17.2 | 21.7 | 23.2 | 22.7 | 23.7 | 21.3 | 19.2 | 17.7 | 18.5 | 15.1 | 9.4 | 5.1 | <0.001 |
| Smoking during pregnancy, % | 11.9 | 6.2 | 3.9 | 3.5 | 3.8 | 4.0 | 4.0 | 4.9 | 5.2 | 5.3 | 7.3 | 10.0 | 15.8 | <0.001 |
| Passive smoking during pregnancy: ≥1 time/week, % | 49.2 | 39.7 | 33.4 | 33.2 | 34.0 | 33.6 | 35.7 | 36.8 | 37.7 | 37.9 | 42.0 | 45.4 | 51.7 | <0.001 |
| Drinking during pregnancy, % | 4.4 | 4.1 | 4.0 | 3.7 | 3.6 | 3.7 | 3.6 | 3.6 | 3.5 | 3.8 | 3.9 | 4.2 | 5.2 | 0.173 |
| Physical activity during pregnancy, METs*min/day | 307 | 283 | 272 | 249 | 245 | 228 | 225 | 214 | 209 | 215 | 210 | 237 | 270 | <0.001 |
| Household income: <4 million yen/year, % | 57.9 | 50.4 | 42.8 | 38.8 | 36.6 | 35.7 | 36.0 | 36.4 | 34.9 | 37.9 | 38.0 | 41.2 | 46.1 | <0.001 |
| Having job during pregnancy, % | 63.9 | 58.9 | 56.7 | 52.7 | 49.4 | 49.0 | 47.4 | 46.5 | 43.8 | 43.3 | 44.1 | 44.0 | 36.7 | <0.001 |
| Having spouse during pregnancy, % | 99.0 | 99.1 | 99.4 | 99.4 | 99.1 | 99.0 | 98.8 | 98.6 | 98.8 | 97.4 | 96.6 | 94.1 | 89.0 | <0.001 |
| Number of children: ≥2 children, % | 37.4 | 32.6 | 29.7 | 30.8 | 31.4 | 33.1 | 34.0 | 37.6 | 37.9 | 41.2 | 44.8 | 45.4 | 46.5 | <0.001 |
| Stress during pregnancy: K6 ≥13 points, % | 4.8 | 3.6 | 2.5 | 2.8 | 2.6 | 2.8 | 2.4 | 2.8 | 3.3 | 3.9 | 3.5 | 4.2 | 5.1 | <0.001 |
| Dietary intake of iron during pregnancy, mg/day | 6.7 | 6.8 | 6.9 | 6.8 | 7.0 | 6.9 | 6.9 | 6.8 | 6.8 | 6.8 | 6.8 | 6.8 | 6.8 | 0.500 |
| Use of anemia medication during pregnancy, % | 14.7 | 12.3 | 11.4 | 12.9 | 13.8 | 14.4 | 14.8 | 15.9 | 16.7 | 16.4 | 15.9 | 16.9 | 16.6 | <0.001 |
| Adequate intake of folate and/or folic acid, % | 23.2 | 26.7 | 28.0 | 28.9 | 29.3 | 29.8 | 30.5 | 29.4 | 27.8 | 28.2 | 27.0 | 28.7 | 28.5 | 0.397 |
| Spontaneous pregnancy, % | 99.4 | 98.7 | 97.6 | 96.6 | 96.2 | 95.1 | 95.4 | 95.3 | 93.8 | 94.2 | 94.6 | 95.3 | 95.4 | <0.001 |
| Pregnancy outcome of last pregnancy: stillbirth, miscarriage or abortion, % | 4.0 | 4.9 | 6.4 | 10.9 | 14.1 | 18.1 | 19.5 | 22.2 | 21.6 | 26.9 | 27.4 | 32.9 | 38.2 | <0.001 |
| Previous history of preterm birth, % | 5.1 | 5.0 | 5.0 | 5.3 | 5.1 | 6.2 | 5.5 | 6.4 | 6.3 | 7.0 | 7.5 | 8.2 | 6.8 | <0.001 |
| Hypertensive disorder of pregnancy, % | 1.6 | 1.7 | 1.6 | 1.6 | 1.9 | 2.1 | 2.4 | 2.2 | 2.2 | 3.3 | 3.5 | 4.2 | 5.8 | <0.001 |
| Gestational diabetes, % | 2.1 | 2.2 | 2.1 | 2.5 | 2.4 | 2.7 | 3.2 | 2.7 | 3.5 | 3.2 | 3.8 | 4.3 | 4.8 | <0.001 |
| Birthweight: <2,500 g % | 8.5 | 6.7 | 5.9 | 6.9 | 7.2 | 7.7 | 7.5 | 7.4 | 7.4 | 7.7 | 8.3 | 9.1 | 11.2 | <0.001 |
| Gestational age at birth, weeks | 38.6 | 38.7 | 38.7 | 38.7 | 38.7 | 38.7 | 38.7 | 38.6 | 38.7 | 38.6 | 38.6 | 38.5 | 38.4 | <0.001 |
| Early preterm birth (22 to <34 weeks), % | 1.8 | 0.8 | 0.6 | 0.8 | 0.8 | 0.9 | 0.8 | 1.1 | 1.0 | 0.8 | 1.1 | 1.8 | 2.2 | <0.001 |
| Late preterm birth (34 to <37 weeks), % | 5.4 | 3.6 | 3.6 | 3.7 | 3.2 | 3.5 | 3.4 | 3.7 | 3.3 | 4.3 | 4.2 | 5.1 | 6.6 | <0.001 |
K6, Kessler Psychological Distress Scale; METs, metabolic equivalents.