Placenta Previa

Placenta previa is placental tissue implanted over the internal cervical os. A low-lying placenta has its lower edge within 20 mm of the os but does not cover it. The distinction matters because a placenta over the os obstructs the birth pathway and is much more likely to bleed when the lower uterine segment stretches or the cervix begins to change. [1] The lower uterine segment does not contract as effectively as the upper uterus after placental separation. That is why bleeding from this location can be brisk and may not respond fully to uterotonic medication alone. The bleeding is often painless and bright red, but the absence of pain does not indicate a minor haemorrhage. Placenta previa is more common after previous caesarean delivery. The combination of previa and a uterine scar also raises concern for placenta accreta spectrum, in which the placenta is abnormally adherent to or invasive into the uterine wall. This possibility changes where and by whom delivery should be planned. [1]