What is the definition and purpose of menstruation (Hayd)?

Primary text

Menstruation is the blood expelled by the uterus upon a woman reaching maturity. Its purpose is intrinsically linked to the wisdom of child-rearing, as this blood is diverted by God's permission to nourish the fetus when pregnancy occurs, hence pregnant women do not menstruate. After childbirth, God Almighty transforms it, by His wisdom, into milk for the infant's nourishment, which is why nursing women rarely menstruate. When a woman is neither pregnant nor nursing, this blood has no recourse and settles, emerging typically for six or seven days each month, though it may increase or decrease, and the cycle length varies according to the natural constitution established by God.