Table 1 Characteristics of mother-infant pairs included and excluded in the study
Variablesa | Participants included (N = 300) n (%) | Participants excluded (N = 925) n (%) | P-value of Student’s t-test or Chi-square test |
|---|---|---|---|
Maternal age at birth (years, mean ± SD) | 27.5 ± 3.5 | 27.9 ± 3.4 | 0.0592 |
Maternal pre-pregnant BMI (kg/m2) | |||
 < 18.5 | 63 (21.4) | 181 (19.9) | 0.8633 |
 18.5–24.9 | 218 (73.9) | 683 (75.1) |  |
 ≥ 25.0 | 14 (4.7) | 45 (5.0) |  |
Parity | |||
 Nulliparous | 261 (87.0) | 767 (83.7) | 0.1744 |
 Multiparous | 39 (13.0) | 149 (16.3) |  |
Maternal education | |||
 High school or less | 68 (22.7) | 226 (24.5) | 0.5460 |
 College or higher | 231 (77.3) | 698 (75.5) |  |
Per capita household income (CNY/month) | |||
 < 4000 | 66 (22.1) | 187 (20.5) | 0.2290 |
 4000–8000 | 130 (43.5) | 359 (39.5) |  |
 > 8000 | 103 (34.4) | 364 (40.0) |  |
Maternal passive smoking before conception | |||
 No | 171 (57.2) | 556 (60.3) | 0.3406 |
 Yes | 128 (42.8) | 366 (39.7) |  |
Gestational age (weeks) | |||
 ≥ 37 | 295 (98.3) | 884 (95.7) | 0.0332 |
 < 37 | 5 (1.7) | 40 (4.3) |  |
Type of delivery | |||
 Vaginal | 154 (51.7) | 368 (46.5) | 0.1246 |
 Cesarean section | 144 (48.3) | 424 (53.5) |  |
Child’s sex | |||
 Male | 167 (55.6) | 500 (54.2) | 0.6642 |
 Female | 133 (44.4) | 422 (45.8) |  |