Objective: To estimate the strength of association between first-trimester bleeding and miscarriage, setting aside bleeding at time of loss.
Methods: Women enrolled in a community-based pregnancy cohort study before or during early pregnancy. Detailed first-trimester bleeding data were collected by telephone interview. Bleeding episodes proximal to miscarriage (within 4 days) were excluded. We used discrete-time hazard models to evaluate the association between bleeding and miscarriage. Models were adjusted for maternal age, prior miscarriage, and smoking. Exploratory regression tree analysis was used to evaluate the relative importance of other bleeding characteristics (duration, associated pain, color, timing).
Results: Of the 4,510 participants, 1,204 (27%) reported some first-trimester vaginal bleeding or spotting, and 517 miscarriages were observed. Eight percent of those with bleeding reported heavy bleeding episodes. When we evaluated any bleeding, including episodes of only spotting, the unadjusted relative odds ratio (OR) of miscarriage for women with bleeding (n=1,204) was 1.1 (95% confidence interval [CI] 0.9-1.3). However, women who reported heavy bleeding (n=97) had nearly three times the risk of miscarriage compared with women without bleeding during the first trimester (OR 3.0, 95% CI 1.9-4.6). Adjustment for covariates had little effect on estimates. Further analyses suggested that women with heavy bleeding accompanied by pain were the group accounting for most of the elevated risk.
Conclusion: Heavy bleeding in the first trimester, particularly when accompanied by pain, is associated with higher risk of miscarriage. Spotting and light episodes are not, especially if lasting only 1-2 days.
Level of evidence: II.