Sex of child
1 = boy
2 = girl

1 = Han
2 = Minorities

1 = 1-11
2 = 12-23
3 = 24-35
4 = 36-47
5 = 48-59

preterm
0 = No
1 = Yes

1 = No
2 = Yes

Maternal education
1 = none and primary
2 = secondary
3 = higher

Family economic status
1 = poorest
2 = poor
3 = middle
4 = richer
5 = richest