Table 1 Characteristics of the study population at delivery and at 4 years of age follow-up
| Â | At delivery N = 415 | 4 year N = 306 | p-value* |
|---|---|---|---|
Parents: | Â | Â | Â |
   Maternal age at birth, mean (SD) | 29.1 (5.4) | 29.8 (5.3) | NS |
   Father's social class, (%) |  |  |  |
Professional | 6.2 | 7.0 | NS |
Managerial and technical | 10.7 | 10.9 | NS |
Skilled (non-manual) | 40.9 | 38.5 | NS |
Skilled (manual) | 22.7 | 24.1 | NS |
Partly skilled | 16.9 | 16.0 | NS |
Unskilled | 2.6 | 3.5 | NS |
   Maternal country, Spain (%) | 74.5 | 81.0 | 0.01 |
Neonates: | Â | Â | Â |
   Sex, male (%) | 51.1 | 53.1 | NS |
   Birth weight (g), mean (SD) | 3242.0 (481.7) | 3180.4 (492.5) | NS |
   Length (cm), mean (SD) | 49.2 (2.1) | 49.1 (2.4) | NS |
   Craneal perimeter (cm), mean (SD) | 34.4 (1.5) | 34.2 (1.6) | NS |
   Low weight (<2500 g), (%) | 6.3 | 8.2 | NS |
   Premature (<37 weeks), (%) | 5.8 | 7.8 | NS |