In this condition, the placenta implants in the lower uterus, close to or even covering the uterine opening, called the internal cervical os, and it can therefore easily bleed, which usually happens after 20 weeks of gestation.

这种情况下，胎盘附着在子宫下部，接近甚至覆盖子宫开口（宫颈内口），导致这个部位在孕期20周后经常会有血液流出。