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What should I know about my monthly cycle (menstrual cycle)?

About once each month during her reproductive years, a woman has a few days when a bloody fluid leaves her womb and passes through her vagina and out of her body. This is called ‘monthly bleeding’, the ‘monthly period’ or ‘menstruation’. It is a healthy process and is part of the way the body gets ready for pregnancy.

Most women think of their monthly bleeding as a normal part of their lives. But often they do not know why it happens or why it sometimes changes.

The monthly cycle is different for each woman. It begins on the first day of a woman’s monthly bleeding. Most women bleed every 28 days. But some bleed as often as every 20 days or as little as every 45 days. A woman may find that the time between each monthly bleeding changes as she grows older, after she gives birth, or because of stress.

The amount of the hormones estrogen and progesterone produced in the ovaries changes throughout the monthly cycle. During the first half of the cycle, the ovaries make mostly estrogen, which causes a thick lining of blood and tissue to grow in the womb. The body makes the lining so a baby would have a soft nest to grow in if the woman became pregnant that month.

About 14 days before the end of the cycle, when the soft lining is ready, an egg is released from one of the ovaries. This is called ovulation. The egg then travels down a tube into the womb. At this time a woman is fertile and she can become pregnant. If the woman has had sex recently, the man’s sperm may join with her egg. This is called fertilization and is the beginning of pregnancy.

During the last 14 days of the cycle - until her next monthly bleeding starts - a woman also produces progesterone. Progesterone causes the lining of the womb to prepare for pregnancy. Most months, the egg is not fertilized, so the lining inside the womb is not needed. The ovaries stop producing estrogen and progesterone, and the lining begins to break down. When the lining inside the womb leaves the body during the monthly bleeding, the egg comes out too. This is the start of a new monthly cycle. After the monthly bleeding, the ovaries start to make more estrogen again, and another lining begins to grow.