Table 1.
Characteristics of the participants according to delivery year
| Characteristics | 2013 | 2014 | 2015 | 2016 | 2017 | 2018 |
|---|---|---|---|---|---|---|
| N | 1,360,384 | 1,813,753 | 1,552,952 | 1,706,939 | 1,709,637 | 1,254,380 |
| Age | ||||||
| 20– | 449,607 (33.1) | 482,363 (26.6) | 391,365 (25.2) | 350,263 (20.5) | 283,488 (16.6) | 205,376 (16.4) |
| 25– | 648,870 (47.7) | 948,058 (52.3) | 818,991 (52.7) | 884,704 (51.8) | 770,323 (45.1) | 563,423 (44.9) |
| 30– | 199,323 (14.7) | 305,331 (16.8) | 257,309 (16.6) | 339,758 (19.9) | 419,649 (24.6) | 337,983 (26.9) |
| ≥ 35 | 62,584 (4.6) | 78,001 (4.3) | 85,287 (5.5) | 132,214 (7.8) | 236,177 (13.8) | 147,598 (11.8) |
| Higher education | ||||||
| Yes | 436,700 (32.1) | 625,611 (34.5) | 573,043 (36.9) | 708,332 (41.5) | 715,633 (41.9) | 581,466 (46.4) |
| No | 891,977 (65.6) | 1,134,400 (62.5) | 927,246 (59.7) | 926,811 (54.3) | 913,797 (53.5) | 600,435 (47.9) |
| NA | 31,707 (2.3) | 53,742 (3.0) | 52,663 (3.4) | 71,796 (4.2) | 80,207 (4.7) | 72,479 (5.8) |
| BMI | ||||||
| Underweight | 191,759 (14.1) | 267,236 (14.7) | 227,708 (14.7) | 232,675 (13.6) | 209,234 (12.2) | 159,800 (12.7) |
| Normal weight | 876,038 (64.4) | 1,147,895 (63.3) | 979,551 (63.1) | 1,051,466 (61.6) | 1,026,015 (60.0) | 750,469 (59.8) |
| Overweight | 169,411 (12.4) | 228,695 (12.6) | 200,350 (12.9) | 234,272 (13.7) | 255,224 (14.9) | 183,551 (14.6) |
| Obese | 102,921 (7.6) | 153,108 (8.4) | 133,190 (8.6) | 176,100 (10.3) | 207,396 (12.1) | 152,599 (12.2) |
| NA | 20,255 (1.5) | 16,819 (0.9) | 12,153 (0.8) | 12,426 (0.7) | 11,768 (0.7) | 7961 (0.6) |
| Region | ||||||
| Northeast | 17,087 (1.3) | 21,362 (1.2) | 12,880 (0.8) | 17,295 (1.0) | 16,405 (1.0) | 13,631 (1.1) |
| North | 70,111 (5.2) | 132,151 (7.3) | 83,421 (5.4) | 113,796 (6.7) | 99,505 (5.8) | 72,659 (5.8) |
| Northwest | 84,483 (6.2) | 104,363 (5.8) | 92,596 (6.0) | 101,283 (5.9) | 108,509 (6.4) | 88,936 (7.1) |
| East | 254,361 (18.7) | 479,177 (26.4) | 339,419 (21.9) | 414,585 (24.3) | 423,651 (24.8) | 316,769 (25.3) |
| Central | 554,208 (40.7) | 657,323 (36.2) | 637,957 (41.1) | 656,494 (38.5) | 625,204 (36.6) | 441,871 (35.2) |
| South | 250,558 (18.4) | 275,153 (15.2) | 253,124 (16.3) | 260,102 (15.2) | 268,554 (15.7) | 180,011 (14.4) |
| Southwest | 129,576 (9.5) | 144,224 (8.0) | 133,555 (8.6) | 143,384 (8.4) | 167,809 (9.8) | 140,503 (11.2) |
| Nationality | ||||||
| Han | 1,242,454 (91.3) | 1,662,843 (91.7) | 1,412,724 (91.0) | 1,562,677 (91.6) | 1,556,050 (91.0) | 1,129,277 (90.0) |
| Others | 102,498 (7.5) | 132,018 (7.3) | 122,268 (7.9) | 120,483 (7.1) | 131,382 (7.7) | 100,268 (8.0) |
| NA | 15,432 (1.1) | 18,892 (1.0) | 17,960 (1.2) | 23,779 (1.4) | 22,205 (1.3) | 24,835 (2.0) |
| Adverse pregnancy outcomes in previous deliveries | ||||||
| Yes | 224,699 (16.5) | 298,746 (16.5) | 241,003 (15.5) | 284,002 (16.6) | 337,703 (19.8) | 253,686 (20.2) |
| No | 1,124,249 (82.6) | 1,505,067 (83.0) | 1,302,460 (83.9) | 1,414,285 (82.9) | 1,363,499 (79.8) | 994,880 (79.3) |
| NA | 11,436 (0.8) | 9940 (0.6) | 9489 (0.6) | 8652 (0.5) | 8435 (0.5) | 5814 (0.5) |
| Parity | ||||||
| Primipara | 1,004,419 (73.8) | 1,254,131 (69.2) | 1,036,872 (66.8) | 1,026,506 (60.1) | 800,238 (46.8) | 610,610 (48.7) |
| Multipara | 343,741 (25.3) | 548,999 (30.3) | 506,195 (32.6) | 671,455 (39.3) | 900,763 (52.7) | 637,945 (50.9) |
| NA | 12,224 (0.9) | 10,623 (0.6) | 9885 (0.6) | 8978 (0.5) | 8636 (0.5) | 5825 (0.5) |
| Full-term births | ||||||
| Yes | 1,234,240 (90.7) | 1,659,942 (91.5) | 1,441,070 (92.8) | 1,593,940 (93.4) | 1,629,940 (95.3) | 1,216,586 (97.0) |
| No | 126,143 (9.3) | 153,809 (8.5) | 111,882 (7.2) | 112,999 (6.6) | 79,696 (4.7) | 37,794 (3.0) |
| NA | 1 (0) | 2 (0) | 0 (0) | 0 (0) | 1 (0) | 0 (0) |
| Number of foetus | ||||||
| Singleton | 1,352,364 (99.4) | 1,802,094 (99.4) | 1,543,985 (99.4) | 1,697,149 (99.4) | 1,700,026 (99.4) | 1,246,359 (99.4) |
| Multiple | 8019 (0.6) | 11,657 (0.6) | 8967 (0.6) | 9790 (0.6) | 9602 (0.6) | 8021 (0.6) |
| NA | 1 (0) | 2 (0) | 0 (0) | 0 (0) | 9 (0) | 0 (0) |
Data are n (%). NA not available, BMI body mass index. BMI was calculated using weight/height2 (kg/m2). BMI < 18.5 was considered underweight, BMI ≥ 18.5 and < 24 were considered normal, BMI ≥ 24 and < 28 were considered overweight, and BMI ≥ 28 was considered obese according to the Chinese population standards. Higher education level was defined as senior high school, college, or postgraduate. Primiparity and multiparity were defined as parity of 0 and ≥ 1, respectively