First-trimester bleeding characteristics associate with increased risk of preterm birth: data from a prospective pregnancy cohort.

BACKGROUND: Prior evidence linking first-trimester bleeding with preterm birth (PTB, <37 weeks gestation) risk has been inconsistent and may be biased by subject selection and/or incomplete documentation of bleeding episodes for all participants. Prior studies have not carefully examined the role of bleeding characteristics in PTB risk. In the present study, we estimate the association between first-trimester bleeding and PTB in a non-clinical prospective cohort and test whether bleeding characteristics better predict risk.
METHODS: Women were enrolled in Right from the Start (2000-2009), a prospective pregnancy cohort. Data about bleeding and bleeding characteristics were examined with logistic regression to assess association with PTB.
RESULTS: Among 3978 pregnancies 344 were PTB and 3634 term. Bleeding was reported by 986 (26%) participants. After screening candidate confounders, only multiple gestations remained in the model. Bleeding associated with PTB [odds ratio (OR)(adjusted) = 1.40, 95% confidence interval (CI) 1.09-1.80]. Risk did not vary by race/ethnicity. Compared with non-bleeders, PTB risk was higher for bleeding with red color (OR(adjusted) = 1.92, 95% CI, 1.32-2.82), for heavy episodes (OR(adjusted) = 2.40, 95% CI 1.18-4.88) and long duration (OR(adjusted) = 1.67, 95% CI 1.17-2.38).
CONCLUSIONS: Bleeding associated with PTB was not confounded by common risk factors for bleeding or PTB. PTB risk was greatest for women with heavy bleeding episodes with long duration and red color and would suggest that combining women with different bleeding characteristics may affect the accuracy of risk assessment. These data suggest a candidate etiologic pathway for PTB and warrant further investigation of the biologic mechanisms.