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 | Â |
Birth weight (g) | 2525 ± 514 |
Gestational age, weeks | 36.8 ± 2.3 |
Birth year | 1986 ± 2.5 |
Sex | Â |
 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 | Â |
 Twin with 1 individual in study | 27 (7.0) |
 Twin with 2 individuals in study | 348 (93.0) |
Childhood | Â |
Age | 11.5 ± 1.6 |
Intelligence score (IQ) | Â |
 Total IQ | 106.0 ± 15.1 |
 Verbal | 106.6 ± 14.6 |
 Performance | 103.9 ± 15.3 |