Placenta previa

The placenta is the organ that forms inside the uterus during pregnancy. In placenta previa, the placenta ends up in the wrong spot: it blocks, fully or partly, the internal cervical os. That's the opening to the cervix, the narrow passage where the uterus opens into the vagina. Picture a room whose only door is blocked from the inside: you can no longer get out that way safely. That's why a vaginal delivery isn't safe and the baby has to be born by cesarean section, the surgery that delivers the baby through a cut in the abdomen and the uterus.

So how does the placenta end up there? The underlying cause isn't known, but placenta previa has been linked to scars in the uterus. The clue lies in implantation, the moment the fertilized egg attaches to the wall of the uterus. To do that, it needs ground that's rich in oxygen and in collagen, a protein that gives tissues their support, and a scar in the uterus offers that kind of ground too. The outer layer of the fertilized egg is called the trophoblast, and it's the part that later becomes the placenta. The trophoblast can latch onto the scar, like a seed taking root in whatever soil it lands on. And that's how the placenta can end up covering the cervical opening.

Most of the time, this is found early on an ultrasound, the test that uses sound waves to see inside the body. Other times, the first sign is vaginal bleeding that doesn't hurt and shows up in the second or third trimester of pregnancy.