Table 1. Time-invariant characteristics of study subjects at 2.5 years old.
| Boys | Girls | Total | ||
| n = 18,013 | n = 16,636 | n = 34,649 | ||
| Birthweight, ga | <2,500 | 1,370 (7.6%) | 1,583 (9.5%) | 2,953 (8.5%) |
| 2,500–2,999 | 5,968 (33.1%) | 6,738 (40.5%) | 12,706 (36.7%) | |
| 3,000–3,499 | 8,039 (44.6%) | 6,708 (40.3%) | 14,747 (42.6%) | |
| ≥3,500 | 2,630 (14.6%) | 1,604 (9.6%) | 4,234 (12.2%) | |
| Missing | 6 (0.0%) | 3 (0.0%) | 9 (0.0%) | |
| Breastfeeding exclusivenessb | Exclusive breastfeeding | 3,810 (21.2%) | 3,668 (22.0%) | 7,478 (21.6%) |
| Mixed feeding | 13,142 (73.0%) | 12,045 (72.4%) | 25,187 (72.7%) | |
| Exclusive formula feeding | 1,061 (5.9%) | 923 (5.5%) | 1,984 (5.7%) | |
| Maternal age at birtha | ≤24 | 1,543 (8.6%) | 1,426 (8.6%) | 2,969 (8.6%) |
| 25–29 | 6,140 (34.1%) | 5,671 (34.1%) | 11,811 (34.1%) | |
| 30–34 | 7,338 (40.7%) | 6,746 (40.6%) | 14,084 (40.6%) | |
| ≥35 | 2,992 (16.6%) | 2,793 (16.8%) | 5,785 (16.7%) | |
| Paternal age at birtha | ≤24 | 946 (5.3%) | 857 (5.2%) | 1,803 (5.2%) |
| 25–29 | 4,363 (24.2%) | 4,116 (24.7%) | 8,479 (24.5%) | |
| 30–34 | 6,723 (37.3%) | 6,201 (37.3%) | 12,924 (37.3%) | |
| ≥35 | 5,819 (32.3%) | 5,279 (31.7%) | 11,098 (32.0%) | |
| Missing | 162 (0.9%) | 183 (1.1%) | 345 (1.0%) | |
| Maternal educational levelc | Junior high school | 580 (3.2%) | 558 (3.4%) | 1,138 (3.3%) |
| High school | 6,837 (38.0%) | 6,382 (38.4%) | 13,219 (38.2%) | |
| Junior or career college | 7,557 (42.0%) | 6,741 (40.5%) | 14,298 (41.3%) | |
| University or higher education | 2,542 (14.1%) | 2,506 (15.1%) | 5,048 (14.6%) | |
| Other/missing | 497 (2.8%) | 449 (2.7%) | 946 (2.7%) | |
| Paternal educational levelc | Junior high school | 1,042 (5.8%) | 944 (5.7%) | 1,986 (5.7%) |
| High school | 6,954 (38.6%) | 6,386 (38.4%) | 13,340 (38.5%) | |
| Junior or career college | 2,753 (15.3%) | 2,553 (15.3%) | 5,306 (15.3%) | |
| University or higher education | 6,608 (36.7%) | 6,134 (36.9%) | 12,742 (36.8%) | |
| Other/missing | 656 (3.6%) | 619 (3.7%) | 1,275 (3.7%) | |
aThis information collected from birth records.
bThis information collected from survey at 0.5 years old.
cThis information collected from survey at 1.5 years old.