Hilchos Bikkurim 11:16

If a son is born via C-section, he is exempt from pidyon haben because he didn’t come out through the birth canal. If another son is born after him, he is also exempt because he was preceded by the first son's birth.

Hilchos Bikkurim 11:17

The obligation of pidyon haben takes effect when the baby has completed thirty days of life. This is explicit in Numbers 18:16: “his redemption from the age of one month you shall redeem.” If the baby dies within the thirty days – including the thirtieth day itself – or if he becomes mortally injured, one need not pay the five sela. If the father gave the kohein the redemption money in advance, the kohein should return it. If the baby dies after thirty days, his father must redeem him. If he didn’t give the money to a kohein, he must do so.