As long as she is put apart - See Clarke's note on Leviticus 15:24.

Also thou shall not approach unto a woman,.... Not even a man to his own wife, and much less to another woman:

to uncover her nakedness, as long as she is put apart for her uncleanness; in her monthly courses; and the time of her separation from her husband on that account was seven days, Leviticus 15:19; if a man lay with a woman when in such circumstances, they were both to be cut off from their people, Leviticus 20:18; and such an action is reckoned among sins, and uncleanness of the worst sort, Ezekiel 22:10.

Prohibition of other kinds of unchastity and of unnatural crimes. - Leviticus 18:19 prohibits intercourse with a woman during her uncleanness. טמאה נדּת signifies the uncleanness of a woman's hemorrhage, whether menstruation or after childbirth, which is called in Leviticus 12:7; Leviticus 20:18, the fountain of bleeding. The guilty persons were both of them to be cut off from their nation according to Leviticus 20:18, i.e., to be punished with death.

Also thou shalt not approach unto a woman to uncover her nakedness, as long as she is put {k} apart for her uncleanness.

(k) Or while she has her period.

18:19 As long as she is set apart - No not to thy own wife. This was not only a ceremonial pollution, but an immorality also, whence it is put amongst gross sins, Eze 18:6. And therefore it is now unlawful under the gospel.

18:1-30 Unlawful marriages and fleshly lusts. - Here is a law against all conformity to the corrupt usages of the heathen. Also laws against incest, against brutal lusts, and barbarous idolatries; and the enforcement of these laws from the ruin of the Canaanites. God here gives moral precepts. Close and constant adherence to God's ordinances is the most effectual preservative from gross sin. The grace of God only will secure us; that grace is to be expected only in the use of the means of grace. Nor does He ever leave any to their hearts' lusts, till they have left him and his services.