Table 6.
Characteristics of the Nested Case-Control Study Population, Overall and by Birth Status (Wuhan, China; June 10, 2011–June 9, 2013)
| Covariate | Total (N = 6656) |
PTBs (N = 2393, 36.0%) |
Controls (N = 4263, 64.0%) |
P Value | |||
|---|---|---|---|---|---|---|---|
| (n) | (%) | (n) | (%) | (n) | (%) | ||
| Maternal age, years | 0.01 | ||||||
| ≤ 21 | 259 | 3.9 | 94 | 3.9 | 16 | 3.9 | |
| 22–35 | 6016 | 90.4 | 2107 | 88.0 | 3909 | 91.7 | |
| > 35 | 381 | 5.7 | 192 | 8.0 | 189 | 4.4 | |
| Maternal educational attainment | 0.96 | ||||||
| Some middle school or below | 736 | 11.1 | 264 | 11.0 | 472 | 11.1 | |
| Some high school | 335 | 45.4 | 121 | 5.1 | 214 | 5.0 | |
| Some college | 2546 | 38.3 | 929 | 38.8 | 1617 | 37.9 | |
| Bachelor’s or higher degree | 3023 | 5.0 | 1073 | 44.8 | 1950 | 45.7 | |
| Missing data | 16 | 0.2 | 6 | 0.3 | 10 | 0.2 | |
| Maternal occupation | 0.12 | ||||||
| Professional | 804 | 12.1 | 275 | 11.5 | 529 | 12.4 | |
| Blue-collar worker | 4751 | 71.4 | 1727 | 72.2 | 3024 | 70.9 | |
| Housewife | 1096 | 16.4 | 387 | 16.2 | 709 | 16.6 | |
| Not reported | 5 | 0.1 | 4 | 0.1 | 1 | 0.01 | |
| Gravidity | 0.01 | ||||||
| ≤3 | 6069 | 91.2 | 2209 | 92.3 | 3860 | 90.5 | |
| > 3 | 587 | 8.8 | 184 | 7.7 | 403 | 9.5 | |
| Sex of infant | 0.01 | ||||||
| Male | 3701 | 55.6 | 1431 | 59.8 | 2270 | 53.2 | |
| Female | 2955 | 44.4 | 962 | 40.2 | 1993 | 46.8 | |
| Parity | 0.20 | ||||||
| 1 | 5584 | 83.9 | 2026 | 84.7 | 3558 | 83.5 | |
| ≥2 | 1072 | 16.1 | 367 | 15.3 | 705 | 16.5 | |
| Season of conception | 0.02 | ||||||
| Spring (Mar.–May) | 1576 | 26.2 | 613 | 25.6 | 963 | 22.6 | |
| Summer (June–Aug.) | 1486 | 23.7 | 545 | 22.8 | 941 | 22.1 | |
| Fall (Sept.–Nov.) | 1851 | 22.3 | 631 | 26.4 | 1220 | 28.6 | |
| Winter (Dec.–Feb.) | 1743 | 27.8 | 604 | 25.2 | 1139 | 26.7 | |
| Total household income (RMB)a | 0.31 | ||||||
| < 59,999 | 2986 | 44.9 | 1082 | 45.2 | 1904 | 44.7 | |
| 60,000–83,999 | 2122 | 31.9 | 737 | 30.8 | 1385 | 32.5 | |
| > 84,000 | 1548 | 23.3 | 574 | 24.0 | 974 | 22.8 | |
| SHS exposure | 0.01 | ||||||
| No | 2395 | 36.0 | 811 | 33.9 | 1584 | 37.2 | |
| Yes | 4261 | 64.0 | 1582 | 66.1 | 2679 | 62.8 | |
| Depression during pregnancy | 0.23 | ||||||
| No | 6070 | 91.1 | 2169 | 90.6 | 3901 | 91.5 | |
| Yes | 586 | 8.9 | 224 | 9.4 | 362 | 8.5 | |
| Vaginal bleeding | 0.01 | ||||||
| No | 4988 | 74.9 | 1590 | 66.4 | 3398 | 79.7 | |
| Yes | 1668 | 25.1 | 803 | 33.6 | 865 | 20.3 | |
| Temperature at conception | 0.35 | ||||||
| Low temperature | 355 | 5.3 | 112 | 4.7 | 243 | 5.7 | |
| Normal temperature | 5954 | 89.5 | 2158 | 90.2 | 3796 | 89.1 | |
| High temperature | 295 | 4.4 | 105 | 4.4 | 190 | 4.4 | |
| Missing data | 52 | 0.8 | 18 | 0.7 | 34 | 0.8 | |
a The renminbi (RMB) – the official currency of China.