Like a seal diving underwater, the baby's heartbeat slows during high-pressure contractions.

How does the baby experience the powerful contractions that mark labor? During contractions, "we know the pressure goes up to about 40 or 50 mm of mercury when we put pressure catheters into the uterus. But it's all around the baby. If you dive down to 100 feet, you don't feel any pressure as long as you're equilibrated. And the baby, being full of fluid, would be equilibrated. I don't know how to tell if there's discomfort for the baby, because it would be like diving down underwater," explains Dr. Copel.

When the membranes holding in the amniotic fluid typically break or are broken during labor, "things get tighter. There's less room to move. The space of the uterus is maintained by the fluid, so the volume of the uterus gets a little smaller. How much that is varies quite a bit with whether the head acts as kind of a ball valve to prevent the fluid from escaping. So the membranes rupturing involves varying amounts of fluid actually escaping. But things probably get a little more snug — it feels like more of a hug from the uterus."