Table 1 Comparison of socio-demographic characteristics in the four pre-pregnancy BMI groups
| Â | Pre-pregnancy BMI category | Â | |||
|---|---|---|---|---|---|
Underweight | Normal weight | Overweight | Obese | P-value | |
N (%) | 420 (13.2) | 2292 (72.3) | 401 (12.6) | 59 (1.9) | Â |
Race | Â | Â | Â | Â | 0.020 |
 Han | 401 (95.5) | 2238 (97.6) | 388 (96.8) | 55 (93.2) |  |
 Minorities | 19 (4.5) | 54 (2.4) | 13 (3.2) | 4 (6.8) |  |
Age (years) |  |  |  |  | <  0.001 |
  <  20 | 20 (4.8) | 69 (3.0) | 12 (3.0) | 5 (8.5) |  |
  ~ 20 | 69 (16.4) | 213 (9.3) | 34 (8.5) | 2 (3.4) |  |
  ~ 25 | 224 (53.3) | 1060 (46.2) | 162 (40.4) | 27 (45.8) |  |
  ~ 30 | 94 (22.4) | 679 (29.6) | 118 (29.4) | 17 (28.8) |  |
  ~ 35 | 10 (2.4) | 222 (9.7) | 62 (15.5) | 7 (11.9) |  |
  ≥ 40 | 3 (0.7) | 49 (2.1) | 13 (3.2) | 1 (1.7) |  |
Education level |  |  |  |  | <  0.001 |
 Elementary school or below | 1 (0.2) | 10 (0.4) | 8 (2.0) | 0 (0) |  |
 Middle school | 58 (13.8) | 337 (14.7) | 82 (20.4) | 16 (27.1) |  |
 High school | 78 (18.6) | 462 (20.2) | 99 (24.7) | 16 (27.1) |  |
 College or university | 255 (60.7) | 1309 (57.1) | 196 (48.9) | 24 (40.7) |  |
 Postgraduate degree | 25 (6.0) | 162 (7.1) | 16 (4.0) | 3 (5.1) |  |
 PhD degree | 3 (0.7) | 12 (0.5) | 0 (0) | 0 (0) |  |
Region |  |  |  |  | <  0.001 |
 South | 188 (44.8) | 820 (35.8) | 120 (29.9) | 15 (25.4) |  |
 North | 232 (55.2) | 1472 (64.2) | 281 (70.1) | 44 (74.6) |  |
Census register type | Â | Â | Â | Â | 0.565 |
 Urban | 177 (42.1) | 982 (42.8) | 167 (41.6) | 20 (33.9) |  |
 Rural | 243 (57.9) | 1310 (57.2) | 234 (58.4) | 39 (66.1) |  |
Occupation | Â | Â | Â | Â | 0.030 |
 Unemployed | 123 (29.3) | 647 (28.2) | 139 (34.7) | 20 (33.9) |  |
 Managerial workers | 41 (9.8) | 236 (10.3) | 39 (9.7) | 3 (5.1) |  |
 Professional and technical workers | 64 (15.2) | 372 (16.2) | 58 (14.5) | 10 (16.9) |  |
 Clerical workers | 48 (11.4) | 236 (10.3) | 22 (5.5) | 6 (10.2) |  |
 Merchant or service workers | 76 (18.1) | 449 (19.6) | 77 (19.2) | 5 (8.5) |  |
 Farming or fishing workers | 4 (1.0) | 45 (2.0) | 13 (3.2) | 2 (3.4) |  |
 Others | 64 (15.2) | 307 (13.4) | 53 (13.2) | 13 (22.0) |  |
Family member | Â | Â | Â | Â | 0.087 |
 1 | 3 (0.7) | 30 (1.3) | 4 (1.0) | 0 (0) |  |
 2 | 146 (34.8) | 673 (29.4) | 115 (28.7) | 17 (28.8) |  |
 3 | 86 (20.5) | 519 (22.6) | 93 (23.2) | 8 (13.6) |  |
 4 | 101 (24.0) | 526 (22.9) | 81 (20.2) | 17 (28.8) |  |
 5 | 59 (14.0) | 409 (17.8) | 73 (18.2) | 9 (15.3) |  |
  ≥ 6 | 25 (6.0) | 122 (5.3) | 33 (8.2) | 7 (11.9) |  |
 Missing | 0 (0) | 13 (0.6) | 2 (0.5) | 1 (1.7) |  |
Self-income (10,000 RMB (1410 USD)/year) | Â | Â | Â | Â | 0.010 |
  ≤ 1 | 53 (12.6) | 284 (12.6) | 71 (17.9) | 13 (22.4) |  |
 1–2 | 38 (9.0) | 219 (9.7) | 42 (10.6) | 9 (15.5) |  |
 2–4 | 114 (27.1) | 607 (26.8) | 104 (26.3) | 23 (39.7) |  |
 4–6 | 120 (28.6) | 617 (27.3) | 110 (27.8) | 5 (8.6) |  |
 6–8 | 33 (7.9) | 170 (7.5) | 22 (5.6) | 2 (3.4) |  |
 8–10 | 40 (9.5) | 239 (10.6) | 32 (8.1) | 2 (3.4) |  |
  > 10 | 22 (5.2) | 126 (5.6) | 15 (3.8) | 4 (6.9) |  |
Family-income (10,000 RMB (1410 USD) /year) |  |  |  |  | <  0.001 |
  ≤ 10 | 223 (53.2) | 1249 (55.4) | 261 (65.9) | 42 (73.7) |  |
 10–20 | 144 (34.4) | 719 (31.9) | 97 (24.5) | 10 (17.5) |  |
  > 20 | 52 (12.4) | 288 (12.8) | 38 (9.6) | 5 (8.8) |  |