Table 1.

Characteristics of singleton births in study sample compared with overall population of singleton births at 37-44 weeks of gestational age, California (1996-2006).

	Total Eligible Singleton Births (n = 4,776,090)	Study Sample (n = 3,545,177)
low birth weight (<2,500 grams)	2.3%	2.3%
maternal age (years)
9 to 14	0.1%	0.2%
15 to 19	9.9%	10.2%
20 to 34	74.2%	74.3%
35 to 49	15.8%	15.4%
educational attainment
none to 11th grade	30.2%	31.5%
12th grade	27.6%	27.6%
1-3 years college	19.8%	19.4%
4+ years college	22.4%	21.4%
marital status
married	42.8%	42.0%
not married	22.5%	23.7%
not on form	27.9%	27.4%
missing	6.8%	6.9%
maternal race/ethnicity
Hispanic	49.6%	51.5%
Black (non-Hispanic)	5.8%	6.3%
American Indian/Alaska Native (non-Hispanic)	0.4%	0.3%
Asian Pacific Islander (non-Hispanic)	11.9%	12.0%
Other Race (non-Hispanic)	0.0%	0.0%
White (non-Hispanic)	32.2%	29.6%
missing	0.1%	0.1%
maternal birthplace
Mexico	27.6%	28.6%
other or unknown foreign country	18.5%	19.2%
US and her territories	53.8%	52.1%
missing	0.1%	0.1%
parity
first live birth	39.5%	39.7%
maternal risk factors
anemia, diabetes, hypertension and/or herpes	4.5%	4.4%
none of the above	86.1%	86.4%
missing	9.4%	9.2%
Kotelchuk index
no prenatal care	1.6%	1.8%
inadequate	9.0%	9.0%
intermediate	11.8%	11.7%
adequate	44.1%	43.8%
more than adequate	33.5%	33.7%
insufficient information	0.1%	0.1%

Eligible singleton births include singleton births with a gestational age of 37-44 weeks and information for birth weight, sex, date of birth, maternal educational attainment, parity, and a maternal age of 9 to 49 years old. Study sample includes eligible singleton births within 10 km of an air monitor active throughout pregnancy.