Table 1.
Comparison of baseline characteristics in case and control groups.
| Variables |
Control group (n = 617) |
Case group (n = 592) |
Univariable analysis |
|---|---|---|---|
| Demographic characteristics | |||
| Gender | χ2 = 2.513, P = 0.000 | ||
| Male | 405 (65.6%) | 303 (51.2%) | |
| Female | 212 (34.4%) | 289 (48.8%) | |
| Maternal age at pregnancy onset (≤ 35) | 554 (89.8%) | 523 (88.3%) | χ2 = 0.648, P = 0.421 |
| Maternal education status | |||
| Less than primary or primary | 7 (1.1%) | 87 (14.0%) | Z = −0.353, P = 0.000* |
| Junior high school | 117 (18.9%) | 263 (42.4%) | |
| Senior middle school | 217 (35.0%) | 167 (26.9%) | |
| College or above | 279 (45.0%) | 103 (16.6%) | |
| Residence location (rural areas) | 339 (54.9%) | 430 (72.6%) | χ2 = 40.851, P = 0.000 |
| Family history | |||
| Consanguineous marriages (yes) | 3 (0.5%) | 19 (3.2%) | χ2 = 12.541, P = 0.000 |
| Adverse pregnancy history before this pregnancy | |||
| Spontaneous abortion (yes) | 60 (9.7%) | 65 (11.0%) | χ2 = 0.514, P = 0.474 |
| Induced abortion or labor (yes) | 189 (30.6%) | 233 (39.4%) | χ2 = 10.125, P = 0.001 |
| Fetal death or stillbirth (yes) | 2 (0.3%) | 18 (3.0%) | χ2 = 13.703, P = 0.000 |
| Premature delivery (yes) | 6 (1.0%) | 9 (1.5%) | χ2 = 0.740, P = 0.390 |
| Low birth weight (yes) | 3 (0.5%) | 9 (1.5%) | χ2 = 3.287, P = 0.070 |
| Neonatal death (yes) | 0 | 9 (1.5%) | P = 0.002** |
| Ectopic pregnancy (yes) | 16 (2.6%) | 13 (2.2%) | χ2 = 0.204, P = 0.652 |
| Congenital malformation (yes) | 2 (0.3%) | 7 (1.2%) | χ2 = 3.012, P = 0.083 |
| Hypertension of pregnancy (yes) | 9 (1.5%) | 33 (5.6%) | χ2 = 15.261, P = 0.000 |
| Gestational diabetes mellitus (yes) | 17 (2.8%) | 58 (9.8%) | χ2 = 25.749, P = 0.000 |
| Personal lifestyle in the 3 months before this pregnancy | |||
| Active smoking | 13 (2.1%) | 24 (4.1%) | χ2 = 3.861, P = 0.049 |
| Positive smoking | 227 (36.8%) | 310 (52.4%) | χ2 = 29.682, P = 0.000 |
| Drinking alcohol | 43 (7.0%) | 73 (12.3%) | χ2 = 10.014, P = 0.002 |
| Exposure to environmental risk factors in the 3 months before this pregnancy | |||
| Harmful chemicals | 46 (7.5%) | 112 (18.9%) | χ2 = 34.947, P = 0.000 |
*
Different between groups were tested by the Wilcoxon rank-sum test.
**
Differences between groups were tested by the Fisher's exact test.