Table 1.
Infant and maternal characteristics by gestational age at birth (1973–1979)
| Gestational Age, % |
||||
|---|---|---|---|---|
| 23–31 weeks (N=2,062) | 32–36 weeks (N=25,873) | 37–42 weeks (N=583,322) | ≥43 weeks (N=18,549) | |
| Gender | ||||
| Female | 44.6 | 44.6 | 48.8 | 49.9 |
| Male | 55.4 | 55.4 | 51.2 | 50.1 |
| Multiple gestation status | ||||
| Singleton | 85.4 | 88.4 | 98.8 | 99.4 |
| Twin | 14.6 | 11.6 | 1.2 | 0.6 |
| Maternal age at delivery (years) | ||||
| <20 | 9.0 | 9.0 | 6.3 | 8.4 |
| 20–24 | 30.0 | 29.0 | 30.0 | 34.9 |
| 25–29 | 32.1 | 34.0 | 38.3 | 37.4 |
| 30–34 | 20.9 | 19.9 | 19.4 | 15.8 |
| ≥35 | 8.1 | 8.1 | 6.0 | 3.5 |
| Maternal marital status in 1990 | ||||
| Married/cohabiting | 70.7 | 71.6 | 76.3 | 72.9 |
| Never married | 12.9 | 12.0 | 9.5 | 11.5 |
| Divorced | 15.5 | 15.0 | 13.0 | 14.6 |
| Widowed | 0.9 | 1.4 | 1.2 | 1.0 |
| Maternal education in 1990 (years) | ||||
| Compulsory high school or less (≤9) | 31.9 | 31.2 | 27.0 | 29.5 |
| Practical high school or some theoretical high school (10–11) | 47.2 | 46.6 | 47.2 | 47.5 |
| Theoretical high school and/or college (≥12) | 20.9 | 22.2 | 25.8 | 23.0 |
| Family income in 1990 | ||||
| Lowest quartile | 28.7 | 26.5 | 23.0 | 24.1 |
| Second quartile | 25.3 | 25.5 | 25.8 | 25.3 |
| Third quartile | 24.4 | 25.0 | 25.7 | 25.8 |
| Highest quartile | 21.6 | 23.0 | 25.5 | 24.8 |
| Maternal prescription of thyroid hormones (≥1/year) during follow-up period | 8.2 | 9.3 | 8.9 | 8.6 |