The cord slips ahead of the baby and gets compressed. You lift the presenting part off it with your hand and you do not let go.
The cord slips down past the presenting part and is squeezed between the baby and the pelvis. Blood flow to the fetus stops.
This is measured in minutes. It is one of the true obstetric emergencies.
Almost always at rupture of membranes — the gush of fluid can carry the cord down.
Higher risk when the presenting part is not engaged:
The very first action after any rupture of membranes is to check the fetal heart rate. Not the pad, not the color - the heart rate.
Variable decelerations mean cord compression — VEAL CHOP. A prolapse is the extreme form.
If the cord is outside the body, cover it with a sterile saline-soaked gauze to prevent drying and vasospasm — but do not attempt to replace it.
Knee-chest and Trendelenburg use gravity to shift the baby’s weight away from the pelvic inlet, taking pressure off the cord.
Both are uncomfortable and undignified, and both save the baby. Explain briefly and keep going.