Torah Temimah → בָּבָא קַמָּא י״א ב׳
English: “But you shall surely redeem” — Rami bar Chama taught: since it says you shall redeem the human firstborn, one might say even if torn within thirty days; therefore it says “but” — to exclude.
No pidyon for a child who dies/is killed within thirty days.
“Akh” carves out the case of a newborn who does not survive thirty days.