As the pregnancy progresses, the lower uterine segment grows, and if the placenta's in the lower uterus, this growth disrupts the placental blood vessels, which can cause bleeding.

随着孕程进展，子宫下部不断增长，而倘若胎盘在子宫下部，此时会引起前置胎盘的血管撕裂，进而导致出血。