At a later gestational age, generally in the third trimester and maybe even earlier because the lower uterine segment begins to form, the placental site will experience detachment. As is well known, the site of the placenta is formed from maternal tissue, namely the decidua basalis, which grows to become part of the uterus. By widening the uterine isthmus into the lower uterine segment, the placenta implanted there will more or less experience lacerations due to detachment of the decidua at the placental site. Likewise, when the cervix flattens (effacement) and opens (dilatation), there is a part of the placenta that is detached. At the laceration site, bleeding will occur from the maternal circulation, namely from the intervillus space of the placenta. Due to the phenomenon of the formation of the lower uterine segment, bleeding in placenta previa somehow inevitably occurs (unavoidable bleeding). Bleeding in that place is relatively easier and multiplied because the lower uterine segment and cervix are not able to contract strongly because the muscle elements they have are minimal, with the result that the blood vessels in that place will not be completely closed.
