Table 2.
Baseline characteristics of the study participants
| Baseline characteristics | Total (n, %) | Active smoking in 3 mo before pregnancy |
Passive smoking in 3 mo before pregnancy |
Active smoking in early pregnancy |
Passive smoking in early pregnancy |
||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
|
|
No (n, %)
|
Yes (n, %)
|
P-value
|
No (n, %)
|
Yes (n, %)
|
P-value
|
No (n, %)
|
Yes (n, %)
|
P-value
|
No (n, %)
|
Yes (n, %)
|
P-value
|
|
Sociodemographic characteristics
| |||||||||||||
| Age (years) |
|
|
|
<0.001 |
|
|
<0.001 |
|
|
0.909 |
|
|
<0.001 |
| <25 |
4980 (11.3%) |
4954 (11.4%) |
26 (3.6%) |
|
4386 (11.7%) |
594 (9.1%) |
|
4914 (11.3%) |
66 (10.9%) |
|
4720 (11.5%) |
260 (8.3%) |
|
| 25-29.9 |
19 724 (44.8%) |
19 510 (45.0%) |
214 (29.9%) |
|
16 796 (44.8%) |
2928 (44.9%) |
|
19 460 (44.8%) |
264 (43.7%) |
|
18 280 (44.7%) |
1444 (45.9%) |
|
| 30-34.9 |
14 192 (32.2%) |
13 822 (31.9%) |
370 (51.7%) |
|
12 004 (32.0%) |
2188 (33.5%) |
|
13 990 (32.2%) |
202 (33.4%) |
|
13 150 (32.2%) |
1042 (33.1%) |
|
| ≥35 |
5152 (11.7%) |
5046 (11.6%) |
106 (14.8%) |
|
4338 (11.6%) |
814 (12.5%) |
|
5080 (11.7%) |
72 (11.9%) |
|
4750 (11.6%) |
402 (12.8%) |
|
| Ethnicity |
|
|
|
0.001 |
|
|
0.184 |
|
|
0.311 |
|
|
<0.001 |
| Han |
41 818 (94.9%) |
41 158 (95.0%) |
660 (92.2%) |
|
35 646 (95.0%) |
6172 (94.6%) |
|
41 250 (94.9%) |
568 (94.0%) |
|
38 910 (95.1%) |
2908 (92.4%) |
|
| Minority |
2230 (5.1%) |
2174 (5.0%) |
56 (7.8%) |
|
1878 (5.0%) |
352 (5.4%) |
|
2194 (5.1%) |
36 (6.0%) |
|
1990 (4.9%) |
240 (7.6%) |
|
| Educational level |
|
|
|
<0.001 |
|
|
0.731 |
|
|
0.152 |
|
|
0.046 |
| Junior high school or below |
6900 (15.4%) |
6678 (15.4%) |
222 (31.0%) |
|
5896 (15.7%) |
1004 (15.4%) |
|
6822 (15.7%) |
78 (12.9%) |
|
6378 (15.6%) |
522 (16.6%) |
|
| Senior middle school |
23 164 (52.6%) |
22 760 (52.5%) |
404 (56.4%) |
|
19 752 (52.6%) |
3412 (52.3%) |
|
22 850 (52.6%) |
314 (52.0%) |
|
21 580 (52.8%) |
1584 (50.3%) |
|
| College |
10 820 (24.6%) |
10 730 (24.8%) |
90 (12.6%) |
|
9192 (24.5%) |
1628 (25.0%) |
|
10 654 (24.5%) |
166 (27.5%) |
|
10 026 (24.5%) |
794 (25.2%) |
|
| Master or above |
3164 (7.2%) |
3164 (7.3%) |
0 (0.0%) |
|
2684 (7.2%) |
480 (7.4%) |
|
3118 (7.2%) |
46 (7.6%) |
|
2916 (7.1%) |
248 (7.9%) |
|
|
Obstetric, clinical, and genetic characteristics
| |||||||||||||
| Mode of conception |
|
|
|
<0.001 |
|
|
<0.001 |
|
|
0.181 |
|
|
<0.001 |
| Spontaneous conception |
34 104 (77.4%) |
33 762 (77.9%) |
342 (47.8%) |
|
29 166 (77.7%) |
4938 (75.7%) |
|
33 650 (77.5%) |
454 (75.2%) |
|
31 752 (77.6%) |
2352 (74.7%) |
|
| Assisted conception |
9944 (22.6%) |
9570 (22.1%) |
374 (52.2%) |
|
8358 (22.3%) |
1586 (24.3%) |
|
9794 (22.5%) |
150 (24.8%) |
|
9148 (22.4%) |
796 (25.3%) |
|
| Parity |
|
|
|
0.447 |
|
|
0.679 |
|
|
0.853 |
|
|
0.603 |
| Primipara |
19 562 (44.4%) |
19 234 (44.4%) |
328 (45.8%) |
|
16 680 (44.5%) |
2882 (44.2%) |
|
19 296 (44.4%) |
266 (44.0%) |
|
18 150 (44.4%) |
1412 (44.9%) |
|
| Multipara |
24 486 (55.6%) |
24 098 (55.6%) |
388 (54.2%) |
|
20 844 (55.5%) |
3642 (55.8%) |
|
24 148 (55.6%) |
388 (56.0%) |
|
22 750 (55.6%) |
1736 (55.1%) |
|
| Consanguineous marriage (yes) |
184 (0.4%) |
178 (0.4%) |
6 (0.8%) |
0.079 |
156 (0.4%) |
28 (0.4%) |
0.876 |
172 (0.4%) |
12 (2.0%) |
<0.001 |
164 (0.4%) |
20 (0.6%) |
0.049 |
| History of adverse pregnancy outcomes (yes) |
17 758 (40.3%) |
17 434 (40.2%) |
324 (45.3%) |
0.007 |
15 146 (40.4%) |
2612 (40.0%) |
0.620 |
17 522 (40.3%) |
236 (39.1%) |
0.531 |
16 512 (40.4%) |
1246 (39.6%) |
0.383 |
| Family history of congenital malformations (yes) |
62 (0.1%) |
54 (0.1%) |
8 (1.1%) |
<0.001 |
54 (0.1%) |
8 (0.1%) |
0.672 |
62 (0.1%) |
0 (0.0%) |
0.353 |
52 (0.1%) |
10 (0.3%) |
0.006 |
| Disease and health conditions |
|
|
|
|
|
|
|
|
|
|
|
|
|
| Pre-pregnancy BMI (kg/m2) |
|
|
|
<0.001 |
|
|
0.672 |
|
|
0.738 |
|
|
0.005 |
| <18.5 |
7532 (17.1%) |
7292 (16.8%) |
240 (33.5%) |
|
6382 (17.0%) |
1150 (17.6%) |
|
7434 (17.1%) |
98 (16.2%) |
|
6982 (17.1%) |
550 (17.5%) |
|
| 18.5-23.9 |
29 324 (66.6%) |
28 952 (66.8%) |
372 (52.0%) |
|
25 008 (66.6%) |
4316 (66.2%) |
|
28 916 (66.6%) |
408 (67.5%) |
|
27 300 (66.7%) |
2024 (64.3%) |
|
| 24-27.9 |
6062 (13.8%) |
5976 (13.8%) |
86 (12.0%) |
|
5168 (13.8%) |
894 (13.7%) |
|
5976 (13.8%) |
86 (14.2%) |
|
5566 (13.6%) |
496 (15.8%) |
|
| ≥28 |
1130 (2.6%) |
1112 (2.6%) |
18 (2.5%) |
|
966 (2.6%) |
164 (2.5%) |
|
1118 (2.6%) |
12 (2.0%) |
|
1052 (2.6%) |
78 (2.5%) |
|
| Pre-pregnancy diabetes mellitus (yes) |
322 (0.8%) |
322 (0.7%) |
10 (1.4%) |
0.045 |
248 (0.7%) |
84 (1.3%) |
<0.001 |
328 (0.8%) |
4 (0.7%) |
0.794 |
314 (0.8%) |
18 (0.6%) |
0.221 |
| Personal history of congenital malformations (yes) |
466 (1.1%) |
466 (1.1%) |
0 (0.0%) |
0.005 |
390 (1.0%) |
76 (1.2%) |
0.360 |
464 (1.1%) |
2 (0.3%) |
0.079 |
432 (1.1%) |
34 (1.1%) |
0.900 |
|
Behaviour characteristics
| |||||||||||||
| Taking folic acid in 3 mo before pregnancy or in early pregnancy (yes) |
42 072 (95.5%) |
1942 (4.5%) |
34 (4.7%) |
0.732 |
1670 (4.5%) |
306 (4.7%) |
0.388 |
1942 (4.5%) |
34 (5.6%) |
0.172 |
1784 (4.4%) |
192 (6.1%) |
<0.001 |
| Alcohol drinking in 3 mo before pregnancy (yes) |
894 (2.0%) |
772 (1.8%) |
122 (17.0) |
<0.001 |
798 (2.1%) |
96 (1.5%) |
0.001 |
886 (2.0%) |
8 (1.3%) |
0.216 |
836 (2.0%) |
58 (1.8%) |
0.440 |
| Alcohol drinking in early pregnancy (yes) |
564 (1.3%) |
526 (1.2%) |
38 (5.3%) |
<0.001 |
498 (1.3%) |
66 (1.0%) |
0.036 |
554 (1.3%) |
10 (1.7%) |
0.409 |
514 (1.3%) |
50 (1.6%) |
0.111 |
|
Exposure of environmentally harmful substances
| |||||||||||||
| Exposure to environmental pollution around the dwelling place in three months before pregnancy or in early pregnancy (yes) |
920 (2.1%) |
902 (2.1%) |
18 (2.5%) |
0.422 |
794 (2.1%) |
126 (1.9%) |
0.336 |
892 (2.1%) |
28 (4.6%) |
<0.001 |
858 (2.1%) |
62 (2.0%) |
0.628 |
| Exposure to radioactive hazardous while at work in three months before pregnancy or in early pregnancy (yes) | 1432 (3.3%) | 1416 (3.3%) | 16 (2.2%) | 0.122 | 1220 (3.3%) | 212 (3.2%) | 0.994 | 1414 (3.3%) | 18 (3.0%) | 0.705 | 1328 (3.2%) | 104 (3.3%) | 0.863 |
BMI – body mass index, mo – months