The King-In-Utero: Let’s say a king (or lord) is married, has no children, and has younger siblings. He his wife gets pregnant, there’s much celebration, but then at about 6 months into the pregnancy… the king dies. Who inherits the throne? His younger brother? Or the fetus? How do you go about coronating a king who has yet to be born?

You’ve described a very good scenario for a succession crisis; the widowed queen and her family are going to fight like hell for the rights of the unborn child, but especially if the younger siblings are adult males, they would have a strong political claim. And then there’s going to be a lot of political drama over the health of the mother, the gender of the baby, whether the baby was born legitimate or secrelty carried into the birthing chamber in a warming pan….

Assuming a male child, they probably wouldn’t be coronated until they’re of age, or at the very least until they were no longer an infant, and there would be a Regency.