Once again, ultrasound comes to the rescue to show the membrane that would put that worry to rest. Placental accidents are more likely, too. Placenta previa, wherein the placenta lies over the opening of the womb, blocking the route out for a vaginal delivery, is more likely because there's more placenta there, and there's less space for it to occupy. Placental abruption, a premature tearing away of the placenta, is also more likely than in single pregnancies. This event could result in significant blood loss and danger of transfusion for the mother and mortal risk for the babies.
Delivery poses dangers as well.
The jumbling together of two babies makes for frequent abnormal positioning of one or both twins. Prolapse of an umbilical cord is more frequent with such malpresentations, so therefore it is more frequent with twins. A cord prolapse can cause it to be pinched off, cutting off oxygen to the baby. Even though it is definitely high risk, we have at our disposal tools to watch closely and intervene. It's the most dramatic argument for good prenatal care, because it's a situation where good prenatal care can be so crucial for a good outcome.