low birth weight
0 = bwt > 2500g
1 = bwt < 2500g

1 = white
2 = black
3 = other

smoking status
0 = non-smoking
1 = smoking

history of hypertension
0 = no
1 = yes

weight in kg in four groups
1 = First quartile
2 = Second quartile
3 = Third quartile
4 = Fourth quartile

history of premature labor
0 = No
1 = Yes

0 = None
1 = 1 or more