Okay, I'll take the bait.
"Mayday" is distress.
"Pan" is urgency.
Therefore, if this situation arose, "pan" would be the call to make. However (and it's a big however!) this situation I am sure would be most unlikely to arise since there's no-one daft enough to take a heavily pregnant woman flying (unnecessarily!) in a light aircraft
Jeremy Pratt's "Air Law" mentions that on average 2 or 3 flights in the UK, every year, land with more passengers than they took off with. Apparently it's almost always in Scotland and due to medical air transfers of pregnant women.