Normal Menstrual Cycle

Women show considerable variation in the lengths of their menstrual cycles, and the length of the menstrual cycle differs in different animals.

While cycle length may vary, 28 days is generally taken as representative of the average ovulatory cycle in women. Convention uses the onset of menstrual bleeding to mark the beginning of the cycle, so the first day of bleeding is called "Cycle Day one".

One can divide the menstrual cycle into four phases:

• Menstruation
• Follicular phase
• Ovulation
• Luteal phase