In the Old Testament law, there were many ceremonial laws that seem very strange to us. For example, there are laws of uncleanness that prohibit touching a corpse, declare uncleanness in birth, sexual intercourse, menstruation and semen discharge. These are ritual states of uncleanness, not moral uncleanness. These things are not evil in themselves, but they were matters of ceremonial uncleanness.

In the Old Testament law, God was making distinctions between pure and impure (Lev. 29:24-26). These laws are meant to teach us that we are set aside for holiness. All day long we divide all activities into holy and unholy. We ought to distinguish everything. 

Perhaps the central text that addresses menstruation is Lev. 15:19-23:

“If a woman has a discharge, and the discharge from her body is blood, she shall be set apart seven days; and whoever touches her shall be unclean until evening. 20 Everything that she lies on during her impurity shall be unclean; also everything that she sits on shall be unclean. 21 Whoever touches her bed shall wash his clothes and bathe in water, and be unclean until evening. 22 And whoever touches anything that she sat on shall wash his clothes and bathe in water, and be unclean until evening. 23 If anything is on her bed or on anything on which she sits, when he touches it, he shall be unclean until evening. 24 And if any man lies with her at all, so that her impurity is on him, he shall be unclean seven days; and every bed on which he lies shall be unclean.”

In Lev. 18:19, we encounter the command against sexual intercourse during menstruation:

“Also you shall not approach a woman to uncover her nakedness as long as she is in her customary impurity.’

Then in Lev. 20:18, we find that it results in being cut off from the people:

“If a man lies with a woman during her sickness and uncovers her nakedness, he has exposed her flow, and she has uncovered the flow of her blood. Both of them shall be cut off from their people.”

There are several passages that mention menstruation: Lev. 12:1-5; Lev. 15:25-30; Lev. 20:18; Lev. 30:22; Ezek. 22:10; Gen. 18:19; Gen. 18:11; Gen. 31:35; Isa. 64:6; Mat. 9:20 and Luke 8:43.

Some of the uses of menstruation are metaphorical pictures of spiritual uncleanness. For example, in Isa. 30:22 it is used of something that is despised, “You will also defile the covering of your graven images of silver, and the ornament of your molded images of gold. You will throw them away as an unclean thing; you will say to them, ‘Get away!’” In Lam. 1:17, Jeremiah laments the destruction that has come upon his people. He says, “Jerusalem has become an unclean thing.” In Ezek. 36:17-18 the prophet speaks of Israel’s defilement using the metaphor of menstruation, “Son of man, when the house of Israel dwelt in their own land, they defiled it by their own ways and deeds; to Me their way was like the uncleanness of a woman in her customary impurity. 18 Therefore I poured out My fury on them for the blood they had shed on the land, and for their idols with which they had defiled it.”

It is obvious that in Scripture, menstruation is a figure of uncleanness. 

Further, the meaning of the Hebrew word “Niddah,” and the various instances of it in the Bible give us some sense of why God has given this condition to women and how to understand it. The Theological Wordbook of the Old Testament testifies that there are 29 occurrences of this word translated, “separation,” “filthiness,” “menstruous,” “unclean.” But what kind of uncleanness was it? It was ritual or ceremonial uncleanness, yet without real sin attached to it. It was a symbolic uncleanness.

It is very important that we know how to interpret ceremonial laws. The Bible gives us three kinds of laws: moral, ceremonial and judicial.

How we ought to interpret ceremonial laws is explained in chapter 19 of the Baptist Confession of 1689:

“Besides this law, commonly called moral, God was pleased to give to the people of Israel ceremonial laws, containing several typical ordinances, partly of worship, prefiguring Christ, his graces, actions, sufferings, and benefits; and partly holding forth divers instructions of moral duties, all which ceremonial laws being appointed only to the time of reformation, are, by Jesus Christ the true Messiah and only law-giver, who was furnished with power from the Father for that end abrogated and taken away (Heb. 10:1; Col. 2:17; 1 Cor. 5:7; Col. 2:14-17; Eph. 2:14-16).”

The authors of the confession explain in the final paragraph: 

“Neither are the aforementioned uses of the law contrary to the grace of the Gospel, but do sweetly comply with it, the Spirit of Christ subduing and enabling the will of man to do that freely and cheerfully which the will of God, revealed in the law, requireth to be done.” 

Here's the summary of how we should think of menstruation and the uncleanness of it:

First, menstruation is not sinful. There is no moral violation in it. The uncleanness laws were referring to ritual uncleanness. It is no sin to menstruate in the same way that it is not sinful to touch a dead body, or experience birth, sexual intercourse, menstruation and semen discharge. These all are figures of something greater. They are earthly reminders of uncleanness. They are part of God’s design in nature so that we would always see the greatness of sin so that we would experience the greatness of God’s grace toward sinners.

Second, menstruation is given as a metaphor to help men and women remember that all of their lives are in need of cleansing – even during the time of menstruation. Every moment is for holiness to the Lord. The laws of ritual cleanness for pregnancy, a dead body, and menstruation… All of these point to the fact that while none of those are sinful, they are holy moments sanctified by the Lord alone and cleansed by the sacrifice... In the new covenant, it's the sacrifice of Christ and the work of His Holy Spirit making things holy - particularly the little moments. I think that means diapers, vacuuming, dishes, and all the other little jobs... Is made holy, for "you shall be holy as I am holy"

Third, I would like to suggest also that it is possible that menstruation is also how a woman can be a figure of an unclean church. The apostle Paul made it clear that a wife was a picture of the church in Ephesians 5:22-33. Therefore, when a woman has her period, she would be reminded of her spiritual uncleanness and the uncleanness of the church without Christ. Perhaps the story of the woman with the flow of blood in Mark 5:25-34 gives us insight into how Jesus was interpreting the law. The woman who came to Him on the street had a menstrual flow for 12 years… For twelve years she was unclean and could not function in the community as other women.

Fourth, menstruation presents an opportunity for a husband to love his wife and live with her "in an understanding way." This could be one facet of the fact that a wife is “a weaker vessel,” and how he should “honor” her as such, humbling himself and putting her above him. During these times a husband should be more aware of her in her difficulty and have compassion for her by helping her get through the time of her menstruation. He should be very sensitive to her un-comfortability and pain. Perhaps menstruation was given to draw love from the heart of a husband for his wife in pain. As he considers how to love his wife as his own body, he moderates and submerges his own desires for her comfort. Her condition would make both of them more aware that their only hope is the abundant grace of Christ since her real uncleanness from sin can only be taken away by the shed blood of Jesus Christ. In this way, a husband sees his wife in the shadow of the cross and has mercy on her as Christ has mercy on unclean sinners.

But what about today? Should a husband and wife have sex during menstruation? My opinion is that it may matter to some and not to others. But, husbands and wives should agree together about this matter and be mutually comfortable with the decision. However, Scripture is clear that we are released from the ceremonial law (Rom. 10:4; Gal. 3:24-26; Eph. 2:15). In this sense, these laws do not apply to us today, but we must never forget that in these things God has given us a gift to help us understand God and the way He has wired the world. The existence of menstruation and dead bodies and semen are designed to tell a story. The principle behind them is a testimony. As everything testifies to something, menstruation testifies to how thoroughly sin has invaded our lives and how important it is that during all of the functions of our lives we are seeing our need for the sacrifice for sin that Jesus has provided.

For deeper insight on interpreting the ritual uncleanness laws in Leviticus this I recommend Jay Sklar’s commentary on Leviticus.