The Pulpit Commentary
She shall be unclean seven days. The mother is to be unclean seven days, and after that to be in the blood of her purifying three and thirty days (Lev_12:4). The difference between these two states maybe seen by looking on to Le Lev_15:19-28, and comparing that passage with Lev_15:4 of this chapter. In the first stage, during the seven days, she made all that she touched unclean; in the second stage, during the thirty-three days, she was only required to touch no hallowed thing, nor come into the sanctuary, as she was progressing towards cleanness. The number of days during which she is to be altogether unclean is to be according to the days of the separation for her infirmity, that is, seven days, as in the case of her monthly courses (see Le Lev_15:19). In the eighth day the flesh of his foreskin shall be circumcised. The Levitical legislation recognizes the regulation as to the day of the circumcision made at the time of the covenant with Abraham. "And he that is eight days old (or a son of eight days) shall be circumcised among you, every man child in your generations" (Gen_17:12). Until the days of her purifying be fulfilled. "When in a state of impurity, the Hebrews were forbidden to enter the sanctuary, to keep the Passover, and to partake of holy food, whether of sacrificial meat, of sacred offerings and gifts, or of shew-bread, because the clean only were fit to approach the holy God and all that appertains to him (Lev_7:19-21; Lev_22:3; Num_9:6; Num_18:11; 1Sa_21:5)' (Kalisch).

If she bear a maid child, then she shall be unclean two weeks;… and she shall continue in the blood of her purifying threescore and six days. The reason why the duration of the mother's uncleanness is twice as long at a girl's birth as at a boy's, would appear to be that the uncleanness attached to the child as well as to the mother, but as the boy was placed in a state of ceremonial purity at once by the act of circumcision, which took place on the eighth day, he thereupon ceased to be unclean, anti the mother's uncleanness alone remained; whereas in the case of a girl, both mother and child were unclean during the period that the former was "in the blood of her purifying," and therefore that period had to be doubly long. See Luk_2:20, where the right reading is, "When the days of their purification, according to the Law of Moses, were accomplished." For eight days the infant Saviour submitted to legal uncleanness in "fulfilling all righteousness" (Mat_3:15), and therefore the whole forty days were spoken of as "the days of their purification."

Lev_12:6, Lev_12:7

The previous verses having stated the conditions and the term of continuance of the uncleanness arising from childbirth, the three final verses describe the offerings to be made by the woman for her purification. She shall bring a lamb of the first year for a burnt offering, and a young pigeon, or a turtledove, for a sin offering. Two things are noticeable here: first, that the burnt offering, symbolizing self-devotion, is far more costly and important than the sin offering, which had not to be offered for any individual personal sin, but only for human sin, "which had been indirectly manifested in her bodily condition" (Keil); and secondly, that in this one case the sin offering appears to succeed the burnt offering instead of preceding it. No doubt the changed order is owing to the cause just mentioned; the idea of sin, though it may not be altogether put aside (Gen_3:16), is not to be prominent, as though it were peculiar to the special woman who was purified.

The Pulpit Commentary
If she be not able to bring a lamb. A concession is made to poverty, which in later times appears to have been largely acted on. It was, as we know, taken advantage of by the mother of our Lord (Luk_2:24).