That happens less than 1% of the time. Medically speaking, when a doctor is faced with a situation where two lives are on the line, first they try to save both, if they can't, they will choose who is more likely to survive any situation. Most of the time, that's the mother because the baby would not live outside the womb. 99% of the time when that happens it's an ectopic pregnancy where the embryo is in the fallopian tubes.