Isaiah 64:6
But we are all like an unclean thing,
And all our righteousnesses are like filthy rags;

This is a wonderful, memorable verse. In my 40+ years of ministry and Bible study, I have heard many sermons about this verse and many explanations on the definition of “filthy rags”.
The truth is in the Hebrew word used, which is Strong’s OT:5708 from an unused root meaning to set a period the menstrual flux (as periodical); by implication (in plural) soiling.

Under the Mosaic Law, during this time a woman was considered ceremonially unclean for seven days.
Anyone she touched was unclean.
Anywhere she sat or laid down was unclean.
Anyone touching her bed was unclean.
Anyone touching something she touched was unclean.
Any man having relations with her was unclean for seven days.

Though this blood is hardly different from any other, the Law says she is unclean and therefore the blood flow took on a sense of filthiness.
To the mindset of many, the blood from a cut on a thumb is different than the blood from a woman’s period.

The Bible says that according to the law almost all things are purified with blood, and without shedding of blood there is no remission (Heb 9:22) but not in the case of the monthly shedding of blood of the woman.
Isaiah used this shedding as a comparison to our righteousness.
My righteousness is like those blood soak rags.
The shedding of my blood would do nothing to cleanse me or anyone else.

Although a woman’s monthly flow is deemed unclean by Law and described as filthy, it becomes a different matter when a fertilized egg is implanted in it.
A young girl named Mary became unclean every month under the Law.
God chose her and implanted a fertilized egg in her.
Nine months later she gave birth to Jesus – when the fullness of the time had come, God sent forth His Son, born of a woman, born under the law, to redeem those who were under the law Gal 4:4-5
Jesus stopped the unclean flow as the fertilized egg. He was born of the water and the blood and later shed His own blood as the one single cure to the uncleanness of sin for all people.