What is a Menstrual Cycle?
Simply put, it's the preparation a woman's body makes for pregnancy every month. It's called a cycle because the same hormonal and physical events occur in sequence each time to prepare an egg for fertilization. The average length of a menstrual cycle is 28 days; however, it can be up to a week shorter or longer and still be normal.
Let's take a closer look at menstrual cycles: When a girl reaches about the age of 12, her body has already gone through many changes internally and externally. Menstrual cycles are a final step in puberty and signal the beginning of her ability to bear children. Her brain, ovaries and uterus have established a working relationship to produce an egg each month and a place for it to grow, if it is fertilized.
What Does this Machinery Look Like?
A uterus is located in the female pelvis and is roughly the size and shape of a small pear (2x6 cm). The lower portion opens into the top of the vagina (about a finger length inside) and is called the cervix. From the wider or upper portion of the uterus extend two delicate tubular structures known as Fallopian tubes. Located at the end of each tube is an ovary.
How Does It All Work?
The brain sends a chemical message through the blood stream to the ovaries. These "hormones" tell the ovaries to start readying an egg for ovulation. They also inform the ovaries to produce a powerful female hormone called estrogen. Estrogen has many effects throughout a woman's body, including stimulation of the growth of the inner lining of the uterus. This lining is called the endometrium.
About two weeks after the start of a woman's last period, an egg is released from one of her ovaries. This event is known as ovulation. The released egg is then picked up by one of the Fallopian tubes to await fertilization. Over the next several days, the ovary produces more estrogen and another hormone called progesterone. This substance puts the finishing touches on the uterine lining, making it ready for the arrival and growth of a fertilized egg.