This is an obstetric emergency where the cord passes through the os in front of the presenting part of the baby.
Pressure of the presenting part on the cord may restrict umbilical cord blood flow resulting in acute foetal hypoxia and foetal distress.
It occurs in 1:200-300 births.
This is an indication for immediate caesarian section.
The management between diagnosis and delivery - a matter of minutes if things run smoothly - is critical to the eventual outcome.
There is a foetal mortality of ten to seventeen percent.