Reasons for an Emergency C-Section
The most common reason for an emergency C-section is that the baby's heart rate starts to show signs of fetal distress. This can happen when a mother's blood pressure falls suddenly. This can happen for a number of reasons, including an epidural anesthetic or a lot of maternal bleeding (for instance, when the placenta comes off the uterine wall prematurely—known as a placental abruption).
Placenta previa (in which the placenta is too near the cervical opening) can also cause maternal hemorrhage and an immediate emergency C-section is called for.
One other reason for emergency C-section is if the umbilical cord prolapses out the vagina, cutting off the baby's circulation. (This is very rare but more common with breech presentations).