Niddah 9 - Definitions of a Virgin, Pregnant, Nursing, and Old Women
One of the four types of women who are not given retroactive impurity , even if they see blood, but are only ritually impure from this moment on, is a virgin. However, this type of virgin is someone who never saw blood in her life, not one who never cohabited with a man. Actually, there are three types of virgins: virgin woman, virgin soil, and virgin sycamore tree - and the precise definitions are important for marriage contracts and contracts of sale, but our virgin is of the fourth type. Since Rabbi Meir said that "Any woman whose bloods are abundant, her children are abundant," - it is the absence of this type of virginity that is desirable to the prospective husband.
A pregnant woman has this status when her pregnancy is noticeable. A nursing woman is one who nurses, until she weans the child - these are the words of Rabbi Meir, who says that there is a direct connection between nursing and the abeyance of blood - and therefore it may last even for four or five years. However, the Sages say the the abeyance of blood is the result of having given birth, and lasts no longer than two years.
An old woman is one whom they call "Mother so-and-so," and in today's terminology probably "Granny," and she is not embarrassed. Others say, until she does not resent it any longer. The difference between these two opinions is the time period when she does not resent it, because it's true, but is still embarrassed. The "old woman" of our ruling is one who is old as above, and then misses her three periods.
Jacobus Frederik Sterre De Jong - A sunlit interior with a mother and a baby