Table 1 Study population characteristics
From: Placental mitochondrial DNA content is associated with childhood intelligence
| Parental characteristics | (n = 201) |
| Education | |
| Low | 16 (8.0) |
| Middle | 71 (35.3) |
| High | 114 (56.7) |
| Neighbourhood income, euro | 18,483 ± 2474 |
| Smoking during pregnancy | 16 (8.0) |
| Cigarettes/day during pregnancy | 1.5 ± 3.7 |
| Urban environment within 5 km, % | 32 ± 12 |
| Characteristics of twins | (n = 375) |
| Birth | |
| Birth weight (g) | 2525 ± 514 |
| Gestational age, weeks | 36.8 ± 2.3 |
| Birth year | 1986 ± 2.5 |
| Sex | |
| Male–male | 143 (38.1) |
| Female–female | 131 (34.9) |
| Male–female | 101 (26.9) |
| Zygosity–chorionicity | |
| Dizygotic-dichorionic | 212 (56.5) |
| Monozygotic-dichorionic | 73 (19.5) |
| Monozygotic-monochorionic | 90 (24.0) |
| Twins in final study | |
| Twin with 1 individual in study | 27 (7.0) |
| Twin with 2 individuals in study | 348 (93.0) |
| Childhood | |
| Age | 11.5 ± 1.6 |
| Intelligence score (IQ) | |
| Total IQ | 106.0 ± 15.1 |
| Verbal | 106.6 ± 14.6 |
| Performance | 103.9 ± 15.3 |