| N | 1991–3 | 1994–6 | 1997–9 | 2000–2 | 2003–6 | |
| Births | 194 936 | 187 906 | 172 817 | 165 496 | 227 089 | |
| Maternal age (years), mean (SD) | ||||||
| Upper white-collar workers | I | 31.7 (4.2) | 32.0 (5.1) | 32.3 (4.2) | 32.5 (4.3) | 32.5 (4.3) |
| Lower white-collar workers | II | 29.5 (4.8) | 30.2 (4.7) | 30.6 (5.0) | 30.6 (5.1) | 30.6 (5.1) |
| Blue collar workers | III | 28.1 (5.1) | 28.7 (5.1) | 29.1 (5.2) | 29.2 (5.4) | 29.2 (5.5) |
| Others | O | 27.8 (5.8) | 27.5 (5.7) | 27.8 (5.8) | 28.0 (6.0) | 28.2 (5.9) |
| Total | 29.3 (5.0) | 29.6 (5.1) | 29.9 (5.1) | 29.9 (5.5) | 30.0 (5.5) | |
| Number of previous pregnancies, mean (SD) | ||||||
| Upper white-collar workers | I | 1.2 (1.4) | 1.3 (1.4) | 1.3 (1.5) | 1.3 (1.4) | 1.2 (1.4) |
| Lower white-collar workers | II | 1.4 (1.5) | 1.4 (1.5) | 1.5 (1.7) | 1.5 (1.6) | 1.6 (1.7) |
| Blue collar workers | III | 1.4 (1.4) | 1.5 (1.5) | 1.6 (1.6) | 1.6 (1.6) | 1.6 (1.7) |
| Others | O | 1.9 (2.2) | 1.8 (2.2) | 1.7 (2.2) | 1.6 (2.1) | 1.6 (1.7) |
| Total | 1.5 (1.6) | 1.5 (1.7) | 1.5 (1.7) | 1.5 (1.8) | 1.5 (1.8) | |
| Number of previous births, mean (SD) | ||||||
| Upper white-collar workers | I | 0.9 (1.1) | 0.9 (1.1) | 0.9 (1.1) | 0.9 (1.1) | 0.9 (1.1) |
| Lower white-collar workers | II | 1.0 (1.2) | 1.0 (1.2) | 1.1 (1.3) | 1.1 (1.3) | 1.1 (1.4) |
| Blue-collarworkers | III | 1.0 (1.1) | 1.0 (1.1) | 1.0 (1.2) | 1.0 (1.2) | 1.0 (1.2) |
| Others | O | 1.4 (1.9) | 1.3 (1.9) | 1.2 (1.8) | 1.2 (1.7) | 1.1 (1.7) |
| Total | 1.0 (1.3) | 1.1 (1.3) | 1.1 (1.4) | 1.1 (1.4) | 1.0 (1.4) | |
| Single mother (%) | ||||||
| Upper white-collar workers | I | 3.3 | 6.3 | 5.5 | 7.2 | 5.1 |
| Lower white-collar workers | II | 5.1 | 9.7 | 9.1 | 11.3 | 8.4 |
| Blue-collar workers | III | 9.1 | 15.6 | 14.2 | 17.6 | 14.0 |
| Others | O | 7.3 | 12.2 | 11.8 | 16.8 | 12.5 |
| Total | 6.0 | 10.8 | 10.1 | 13.2 | 9.9 | |
| Smoked during pregnancy (%) | ||||||
| Upper white-collar workers | I | 5.5 | 4.2 | 4.1 | 4.0 | 3.9 |
| Lower white-collar workers | II | 14.2 | 12.6 | 12.1 | 12.7 | 12.8 |
| Blue-collar workers | III | 28.2 | 25.2 | 24.5 | 25.8 | 25.5 |
| Others | O | 18.1 | 18.0 | 17.9 | 18.2 | 18.7 |
| Total | 16.3 | 14.7 | 14.4 | 14.8 | 14.8 | |
| Multiple birth (%) | ||||||
| Upper white-collar workers | I | 1.5 | 1.8 | 2.1 | 2.0 | 1.7 |
| Lower white-collar workers | II | 1.4 | 1.6 | 1.8 | 1.6 | 1.6 |
| Blue-collar workers | III | 1.4 | 1.4 | 1.5 | 1.6 | 1.4 |
| Others | O | 1.2 | 1.2 | 1.4 | 1.3 | 1.4 |
| Total | 1.4 | 1.5 | 1.7 | 1.6 | 1.5 | |
*All differences between the socioeconomic groups are statistically significant, p<0.001.