Table 2. Individual associations between menstrual characteristics and SA risk.
| Characteristic | Total participants |
SA cases n (%) |
OR (95% CI) | |||
| Model 1 | Model 2 | Model 3 | Model 4 | |||
| Note: Data are presented as odds ratio (95% CI). Estimates with 95% CIs excluding 1 are considered statistically significant at P<0.05. Models examined associations between menstrual cycle regularity (regular: 24–38 days; irregular: <24, 39–53, or >53 days) and menstrual period duration (normal: 3–7 days; abnormal: <3 or >7 days) with SA risk. Four models were fitted: Model 1 (unadjusted), Model 2 (adjusted for high-risk factors), and Model 3 (fully adjusted) using conventional logistic regression; Model 4 (fully adjusted) using multilevel logistic regression as an extended analytical approach. All covariates are listed in Supplementary Table S2. Abbreviation: OR=odds ratio; CI=confidence interval; SA=spontaneous abortion. | ||||||
| Menstrual cycle | ||||||
| Binary classification | ||||||
| Regular (24–38 days) | 3,743,087 | 62,165 (1.66) | 1.00 (Reference) | 1.00 (Reference) | 1.00 (Reference) | 1.00 (Reference) |
| Irregular | 117,828 | 2,559 (2.17) | 1.31 (1.26, 1.37) | 1.23 (1.18, 1.29) | 1.11 (1.05, 1.17) | 1.11 (1.05, 1.17) |
| Detailed classification | ||||||
| 24D–38D | 3,743,087 | 62,165 (1.66) | 1.00 (Reference) | 1.00 (Reference) | 1.00 (Reference) | 1.00 (Reference) |
| <24D | 69,579 | 1,454 (2.09) | 1.26 (1.20, 1.33) | 1.22 (1.15, 1.29) | 1.12 (1.04, 1.20) | 1.10 (1.02, 1.17) |
| 39D–53D | 38,707 | 865 (2.23) | 1.35 (1.26, 1.45) | 1.22 (1.13, 1.32) | 1.06 (0.97, 1.17) | 1.09 (0.99, 1.12) |
| >53D | 9,542 | 240 (2.52) | 1.53 (1.34, 1.74) | 1.40 (1.21, 1.61) | 1.25 (1.06, 1.48) | 1.25 (1.06, 1.48) |
| Menstrual period | ||||||
| Binary classification | ||||||
| Regular (3D–7D) | 3,729,330 | 61,780 (1.66) | 1.00 (Reference) | 1.00 (Reference) | 1.00 (Reference) | 1.00 (Reference) |
| Irregular | 61,810 | 1,643 (2.66) | 1.62 (1.54, 1.70) | 1.44 (1.36, 1.52) | 1.23 (1.15, 1.32) | 1.26 (1.17, 1.35) |
| Detailed classification | ||||||
| 3D–7D | 3,729,330 | 61,780 (1.66) | 1.00 (Reference) | 1.00 (Reference) | 1.00 (Reference) | 1.00 (Reference) |
| <3D | 29,729 | 743 (2.50) | 1.52 (1.41, 1.64) | 1.35 (1.25, 1.46) | 1.11 (0.99, 1.25) | 1.18 (1.05, 1.32) |
| >7D | 32,081 | 900 (2.81) | 1.71 (1.60, 1.83) | 1.53 (1.42, 1.64) | 1.31 (1.20, 1.42) | 1.31 (1.20, 1.42) |