Sefer HaChinuch ספר החינוך

The law of the sacrifice of the woman that has given birth: That a woman who has given birth bring a sacrifice when the days of her purification for a son or a daughter are completed. And it is a one-year-old lamb for a burnt-offering and a young pigeon or a dove for a sin-offering. And if she is poor, she brings two doves or two young pigeons — one for a burnt-offering and one for a sin-offering, as it is stated (Leviticus 12:6), “On the completion of the days of purification for a son or daughter, she shall bring, etc.” And she is lacking atonement until she brings her sacrifice.

It is from the roots of the commandment [that it is] in order that she be aroused by action to give thanks to God, blessed be He, Who saved her from the pangs of her birth, which is a miraculous thing. And also they, may their memory be blessed, said (Niddah 31b) that a woman rushes to swear at the time of her throes that she will no longer [have sexual relations with] a man. And therefore she requires atonement, and hence her sacrifice is called a sin-offering, and it is stated about her, “and the priest shall atone for her” (Leviticus 12:8).

From its laws is, for example, that which they, may their memory be blessed, said that a woman that has given birth does not bring her sacrifice on the fortieth day for a male and on the eightieth day for a female, but rather on the morrow, which is the forty-first day, and likewise the eighty-first day, as it is stated, “On the completion of the days of purification” — meaning to say that they are totally completed. And if she brought it during the days of completion, she has not fulfilled her obligation (Zevachim 112b). If the time has passed and she did not bring her atonement, she brings it after the time. And the whole time that she has not brought it, she is forbidden to eat consecrated foods — and as they, may their memory be blessed, said (Mishnah Negaim 4:3), “[When] she brought her atonement, she eats from consecrated foods.” And all those lacking atonement have the same status in this regard. And [regarding] all those that are obligated to bring a sacrifice, we may not bring their sacrifice without their consent; except for [the sacrifice of] those lacking atonement, which does not require consent of the owners (Nedarim 35b). And the rest of its details are in Tractate Niddah. (See Mishneh Torah, Laws of Offerings for Those with Incomplete Atonement 1.)

And [it] is practiced at the time of the [Temple] by females. And one who transgresses it and does not bring her sacrifice has violated this positive commandment, and she is lacking atonement and forbidden to eat from consecrated foods. And woe is to her if she dies before she brings it and [so] carries her iniquity upon her soul.