Here is a photo of a velamentous insertion of the umbilical cord. This is when the vessels of the cord imbed into the chorion and run inbetween the amnion and chorion (the amniotic sac) for some length. Sometimes, the insertion is a couple of centimeters, but others, like this, are quite far.
When the vessels are at or near the opening of the cervix (Vasa Previa), they can burst when the membranes rupture and cause terrible damage or death to the baby. The baby "exsanguinates" - loses all his/her blood within a few minutes. One of my previous clients (x2) was pregnant with her fourth baby, had had three previous homebirths and was expecting to birth this one at home, too, but her membranes spontaneously ruptured and blood poured out. They called 911 and she was taken to the hospital where, a mere 15 or so minutes later, her baby had already died. She was induced and the baby boy was born vaginally. His family spent several hours with him, taking pictures of him and loving him lots before letting him go to be ready for burial. She went on the have two more babies with me at home... beautiful births, but her son was always in our minds.
In Facebook, someone wisely asked what would happen if there was cord traction on this type of placenta. This next picture shows what happens.
The cord falls off.
We were taught Active Management of Third Stage, doing controlled cord traction to get the placenta out in a "timely manner." If there is a velamentous insertion, the cord is tenuously connected, so it doesn't take much to break it off.
With this woman's placenta, we had to go in and manually remove it.
When the cord has come off, I've only seen going in to get the placenta, but always wondered about just letting the placenta be born on its own. Is that possible? If we're going to not do cord traction, even if to get the placenta out of the vaginal vault, would the organ plop out on its own if the woman was upright?