Table 1.

Characteristics of the study population at delivery and at 4 years of age follow-up

	At delivery N = 415	4 year N = 306	p-value*
Parents:
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:
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

NS: no significance