Table 1.

Demographic Characteristics of Study Population by Exposure Period (n=56,155)

	2008 N = 26,451	2009 N = 29,704
	N (%)	N (%)
Maternal age (years, mean ± SD)	29.4 ± 3.6	29.5 ± 3.6
Maternal education
High school diploma or less	4, 275 (16)	4, 080 (14)
Some college or technical school	7, 939 (30)	7, 958 (27)
Bachelor’s degree	14, 237 (54)	17, 666 (59)
District of maternal residence
Chaoyang district	10,491 (40)	12,568 (42)
Fengtai district	5,753 (22)	6,465 (22)
Haidian district	8,540 (32)	8,896 (30)
Xicheng district	1,667 (6)	1,775 (6)
Gestational age at first clinic visit (weeks, mean ± SD)	7.3 ± 3.6	7.1 ± 2.7
Pregnancy complications
Hypertensive disorders	286 (1.1)	347 (1.2)
Gestational hypertension	166 (0.6)	218 (0.7)
Preeclampsia	113 (0.4)	126 (0.4)
Eclampsia	7 (0.0)	3 (0.0)
Fetal-placental conditions	3,088 (11.7)	3,486 (11.7)
Fetal macrosomia	1,620 (6.1)	1,868 (6.3)
Fetal distress	565 (2.1)	570 (1.9)
Placental abruption	23 (0.1)	26 (0.1)
Threatened preterm labor	181(0.7)	231 (0.8)
Polyhydramnios	17 (0.1)	30 (0.1)
Oligohydramnios	226 (0.9)	261 (0.9)
Premature rupture of membranes	456 (1.7)	500 (1.7)