These two phrases mean basically the same thing and are combined for emphasize the firstborn sons. (See: [[rc://en/ta/man/translate/figs-parallelism]])
This is an idiom. "To open the womb" means to give birth for the first time. Here this refers to the first male child to which a mother gives birth. Alternate translation: "child who is the firstborn son of his mother" (See: [[rc://en/ta/man/translate/figs-idiom]])