Charles Ellicott Commentary Leviticus 12:7

"and he shall offer it before Jehovah, and make atonement for her; and she shall be cleansed from the fountain of her blood. This is the law for her that beareth, whether a male or a female." — Leviticus 12:7 (ASV)

Who shall offer it. —Better, and he shall offer it, that is, the priest shall offer the sin offering. Though two sacrifices were brought—a burnt offering and a sin offering—yet stress is laid on the sin offering, for on it depended the purification and atonement of the mother. Even if the mother gave birth to twins, the administrators of the law during the second Temple decided that the one sin offering here prescribed sufficed.