Is the husband sinful for delaying the fulfillment of his vow to slaughter two animals after his wife became pregnant, and is it permissible for him to postpone it until after childbirth?

Making a vow is disliked (makrooh), but fulfilling it is obligatory (wajib). A time-bound vow must be fulfilled at its appointed time. If it is delayed without an excuse, then fulfillment is still required along with expiation for breaking an oath. As long as the husband did not specify a time for the slaughter, he is not obligated to hasten it.