This chapter would more naturally follow the 15th chapter of Leviticus. See the note to Leviticus 15:1.

And the Lord spake unto Moses,.... The laws in the preceding chapter were delivered both to Moses and Aaron, but what follows in this only to Moses; but inasmuch as the priest had a concern in it, it being his business to offer the sacrifices required by the following law, it was no doubt given to Moses, to be delivered to Aaron, as well as to the people. R. Semlai remarks, that as the creation of man was after that of the beasts, fowls, fishes, &c. so the laws concerning the uncleanness of men are after those relating to beasts, &c, and they begin with the uncleanness of a new mother, because, as Aben Ezra observes, the birth is the beginning of man:
saying: as follows.

After the laws concerning clean and unclean food, come the laws concerning clean and unclean persons. Man imparts his depraved nature to his offspring, so that, excepting as the atonement of Christ and the sanctification of the Spirit prevent, the original blessing, "Increase and multiply," Genesis 1:28, is become to the fallen race a direful curse, and communicates sin and misery. Let those women who have received mercy from God in child-bearing, with all thankfulness own God's goodness to them; and this shall please the Lord better than sacrifices.

Uncleanness and Purification after Child-Birth. - Leviticus 12:2-4. "If a woman bring forth (תּזריע) seed and bear a boy, she shall be unclean seven days as in the days of the uncleanness of her (monthly) sickness." נדּה, from נדד to flow, lit., that which is to flow, is applied more especially to the uncleanness of a woman's secretions (Leviticus 15:19). דּותהּ, inf. of דּוה, to be sickly or ill, is applied here and in Leviticus 15:33; Leviticus 20:18, to the suffering connected with an issue of blood.

From uncleanness contracted by the touching or eating of external things, he now comes to that uncleanness which ariseth from ourselves.