Pathophysiology
Clinical meaning
Placenta previa occurs when the placenta implants over or near the opening of the cervix. Normally the placenta implants in the upper uterus. When it covers the cervix, it blocks the baby's exit and can cause severe bleeding. Types include complete previa (placenta completely covers the cervical opening), partial previa (partially covers), and marginal previa (edge near but not covering). Low-lying placenta means it is within 2 cm of the cervix. The hallmark is PAINLESS bright red vaginal bleeding, usually starting in the late second or third trimester. As the cervix thins and dilates, the placenta separates from the cervix, tearing blood vessels. Key difference from abruption: previa causes PAINLESS bleeding with SOFT uterus; abruption causes PAINFUL bleeding with RIGID uterus. Do NOT perform a digital vaginal exam on a patient with suspected placenta previa — this can cause massive hemorrhage.