Baseline and obstetric characteristics of the study subjects
| Characteristic | Week 37 (n=1185) | Week 38 (n=7227) | Week 39 (n=7656) | Week 40 (n=3069) | Week ≥41 (n=802) | p Value |
| Maternal age, years (%) | <0.001 | |||||
| ≥35 | 23.9 | 19.1 | 16.1 | 8.2 | 6.2 | |
| 30~34 | 33.5 | 37.8 | 37.4 | 40.1 | 37.5 | |
| 25–29 | 38.2 | 39.8 | 42.9 | 47.7 | 52.0 | |
| ≤24 | 4.4 | 3.4 | 3.6 | 3.9 | 4.2 | |
| Insurance (%) | 61.9 | 66.5 | 72.8 | 72.5 | 73.9 | <0.001 |
| Married (%) | 99.8 | 99.4 | 99.6 | 99.4 | 99.4 | 0.160 |
| Body mass index at first prenatal visit* | 24.2±3.4 | 23.6±3.0 | 23.3±3.0 | 23.1±3.1 | 22.9±3.2 | <0.001 |
| Education, years† | 14.8±2.7 | 15.0±2.7 | 14.9±2.7 | 15.2±2.8 | 14.8±2.8 | 0.003 |
| Assisted conception (%) | 8.3 | 6.2 | 5.1 | 2.8 | 1.5 | <0.001 |
| GBS (%)‡ | 3.7 | 4.0 | 3.5 | 3.7 | 3.9 | 0.479 |
| STD (%)§ | 1.8 | 1.7 | 1.6 | 1.7 | 1.8 | 0.986 |
| Complicated with other medical disorders (%)¶ | <0.001 | |||||
| No | 80.1 | 84.6 | 86.0 | 89.2 | 93.5 | |
| Yes | 19.9 | 15.4 | 14.0 | 10.8 | 6.5 | |
| Birth weight (%) | <0.001 | |||||
| ≥4000(g) | 2.1 | 3.2 | 5.6 | 9.8 | 11.8 | |
| 3500~3999(g) | 13.3 | 28.4 | 39.1 | 50.6 | 55.6 | |
| 2500~3499(g) | 78.7 | 67.9 | 55.1 | 39.5 | 32.5 | |
| <2500(g) | 5.8 | 0.5 | 0.1 | 0.1 | 0.0 | |
| Male sex (%) | 54.2 | 53.5 | 51.7 | 50.2 | 48.6 | 0.003 |
| First-trimester ultrasound (%) | 95.1 | 94.1 | 94.2 | 94.7 | 93.3 | 0.356 |
Plus–minus values are means±SD.
*Body mass index, is the weight in kilograms divided by the square of the height in metres. Values were missing for 1196 (6.0%) women.
†Values were missing for 1176 (5.9%) women.
‡ 585 (2.9%) women did not have this test.
§In Shanghai, syphilis, gonorrhoea and chlamydia are routinely tested. Values were missing for 40 (0.2%) women.
¶Complicated at least one of medical disorders as cardiac disease, hepatitis, renal disease, diabetes mellitus or gestational diabetes mellitus , chronic hypertension and gestational hypertension, thyroid disease, pre-eclampsia, intrahepatic cholestasis of pregnancy.
GBS, group B streptococcus; STD, sexually transmitted disease.