Table 3. Combined associations between menstrual characteristics and SA risk.
| Menstrual Characteristic Pattern |
Total
participants |
SA cases n (%) |
OR (95% CI) | |||
| Model 1 | Model 2 | Model 3 | Model 4 | |||
| Note: Exposure categories were defined based on combinations of menstrual cycle length and menstrual period duration. The reference group comprised women with both normal cycle length and normal period duration. Both normal: normal menstrual cycle length (24–38 days) and normal period duration (3–7 days). Irregular cycle only: irregular menstrual cycle (<24 or >38 days) with normal period duration. Irregular period only: normal menstrual cycle with irregular period duration (<3 or >7 days). Both abnormal: irregular menstrual cycle and irregular period duration. Four models were constructed: 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. Results are presented as odds ratios (95% CI). Associations are considered statistically significant at P<0.05, corresponding to 95% CIs that exclude 1. Abbreviation: CI=confidence interval; SA=spontaneous abortion. | ||||||
| Binary joint classification | ||||||
| Both normal | 3,657,947 | 60,231 (1.62) | 1.00 (Reference) | 1.00 (Reference) | 1.00 (Reference) | 1.00 (Reference) |
| Irregular cycle only | 67,043 | 1,444 (2.11) | 1.31 (1.25, 1.39) | 1.20 (1.13, 1.27) | 1.10 (1.01, 1.17) | 1.11 (1.03, 1.20) |
| Irregular period only | 56,991 | 1,498 (2.56) | 1.61 (1.53, 1.70) | 1.42 (1.34, 1.50) | 1.24 (1.16, 1.34) | 1.27 (1.18, 1.37) |
| Both abnormal | 4,726 | 140 (2.88) | 1.82 (1.54, 2.16) | 1.45 (1.20, 1.76) | 1.14 (0.90, 1.44) | 1.18 (0.93, 1.49) |
| Detailed joint classification | ||||||
| Both normal | 3,657,947 | 60,231 (1.62) | 1.00 (Reference) | 1.00 (Reference) | 1.00 (Reference) | 1.00 (Reference) |
| Short cycle & short period | 1,414 | 33 (2.28) | 1.43 (1.01, 2.02) | 1.01 (0.67, 1.51) | 0.97 (0.65, 1.47) | 0.68 (0.38, 1.20) |
| Short cycle & regular period | 21,392 | 420 (1.93) | 1.20 (1.09, 1.32) | 1.11 (1.00, 1.23) | 1.07 (0.97, 1.20) | 1.12 (0.97, 1.29) |
| Short cycle & long period | 804 | 30 (3.60) | 2.23 (1.61, 3.33) | 1.75 (1.15, 2.66) | 1.52 (0.99, 2.32) | 1.44 (0.90, 2.31) |
| Regular cycle & short period | 27,655 | 690 (2.43) | 1.53 (1.42, 1.65) | 1.34 (1.24, 1.46) | 1.31 (1.20, 1.42) | 1.22 (1.08, 1.38) |
| Regular cycle & long period | 29,336 | 808 (2.68) | 1.69 (1.58, 1.82) | 1.50 (1.39, 1.62) | 1.39 (1.29, 1.50) | 1.30 (1.19, 1.42) |
| Long cycle & short period | 472 | 15 (3.08) | 1.96 (1.17, 3.28) | 1.36 (0.75, 2.48) | 1.37 (0.75, 2.51) | 1.31 (0.61, 2.79) |
| Long cycle & regular period | 36,704 | 802 (2.14) | 1.33 (1.24, 1.43) | 1.20 (1.11, 1.29) | 1.19 (1.10, 1.29) | 1.07 (0.97, 1.17) |
| Long cycle & long period | 1,463 | 46 (3.05) | 1.94 (1.45, 2.60) | 1.98 (1.46, 2.69) | 1.82 (1.33, 2.49) | 1.64 (1.15, 2.34) |
| Very long cycle & short period | 146 | 2 (1.35) | 0.83 (0.21, 3.35) | 0.78 (0.19, 3.17) | 0.75 (0.18, 3.04) | 0.58 (0.08, 4.21) |
| Very long cycle & regular period | 8,947 | 222 (2.42) | 1.52 (1.33, 1.74) | 1.41 (1.22, 1.63) | 1.40 (1.21, 1.62) | 1.29 (1.08, 1.53) |
| Very long cycle & long period | 427 | 14 (3.17) | 2.02 (1.19, 3.45) | 1.08 (0.51, 2.28) | 1.04 (0.50, 2.21) | 0.59 (0.19, 1.86) |