Your monthly cycle

Your monthly cycle

This may come as a surprise, but your 'monthly' cycle doesn’t necessarily take place once a month. The average menstrual cycle time for most girls is 28 days, but your cycle may last from 21 to 35 days and still be normal. In your cycle, 'day one' is the first day of your period, or the first day you begin to bleed.
If you have a short cycle, you may have a period more often than once a month. However, if your cycle lasts longer, you’re one of the girls who have fewer periods in a year.
In general, a girl's period will start somewhere between the ages of 10 and 13 (on average at age 12). It may take a year or two for a girl’s period to become regular. She will have a period about once a month (except if she is pregnant) until she reaches menopause, the time when her periods stop, typically somewhere around age 51.

Your reproductive organs

When learning about your menstrual cycle, it's helpful to know what parts make up a woman's reproductive organs.
Your reproductive organs include:

  • Two ovaries: this is where eggs (ova) are stored and released. Also, the ovaries produce hormones (Side note: a human egg is tiny; about 120 microns or micrometres when it attains its full size. This is about the same size as the width of a human hair.)
  • The uterus (womb): where a fertilised egg implants and carries a baby until it is born
  • Two fallopian tubes: these are two thin tubes through which eggs travel from the ovaries to the uterus
  • The cervix: the lower part of the uterus that connects to the vagina
  • The vagina: a flexible passageway leading from the uterus to the outside of the body through which menstrual fluid flows. (Side note: your vagina is inside your body – the part of your body on the outside that you can see, is your vulva.)

Who controls your cycle : hormones

Each month, your reproductive system repeats a regular pattern of events that are controlled by hormones – in particular, rising and falling levels of oestrogen and progesterone.
Rising levels of hormones cause the ovary to develop an egg and release it, a phase called ovulation. After ovulation, in preparation for a potential pregnancy, hormones help the lining of the uterus grow thicker.
Each month an egg is released by one of the ovaries and travels down the connecting fallopian tube. If pregnancy doesn’t occur, the egg leaves your body with your menstrual flow. Levels of oestrogen and progesterone fall, and the lining of the uterus – comes away and leaves the body. This is your menstrual flow - your period (also known as menses). The time from the release of the egg to the start of a period is around two weeks.

 

The menstrual cycle

  1. Pre-ovulation phase
    Women and girls have two ovaries that contain thousands of eggs (ova). Hormones tell an ovary to release an egg every month. At the same time, the soft lining (called the endometrium) of the uterus (the place where a baby can grow) starts to thicken.
  2. Ovulation
    This occurs when a mature egg (occasionally two) is released from the ovary. After the egg is released, it travels along the fallopian tube to the thickening lining of the uterus. If sperm from a male fertilises the egg, a baby develops. After ovulation, if fertilisation is to occur, it must happen within 24 hours of ovulation or the egg degenerates. Ovulation usually happens around 2 weeks before the next period.
  3. Post ovulation phase
    After ovulation, the lining of the uterus continues to develop in preparation for the implantation of a fertilised egg. If the egg isn’t fertilised, the hormones that stimulate the development of the uterine lining will stop being produced about two weeks after ovulation. The lining is then shed (your period) starting your next menstrual cycle.
  4. Menstruation
    The lining of the uterus leaves your body through the vagina as a reddish fluid – typically about a quarter of a cup for a period (though it can seem like a lot more). This fluid loss – your period – will last between three to seven days. The first day of bleeding is day one of your period.

All about eggs

  • Over the course of a lifetime, you release about 400 eggs in their mature form
  • The number of eggs that are still contained in the ovaries depends on how old you are
  • As a 20-week-old female foetus in your mother’s uterus, you have the highest number of eggs you will ever have, approximately seven million eggs
  • Your body will release the most eggs it ever will before you are even born
  • At birth, the number of eggs in ovaries drops to two million. Your ovaries will continue to lose eggs after birth all the way through puberty
  • By the time you start puberty, you have between 300,000 and 500,000 eggs in your ovaries
  • Only about 400 eggs will ripen into mature eggs during your lifetime.
Always

Sources

  • The American Congress of Obstetricians and Gynecologists (ACOG), Patient brochure 041

  • ACOG, Factsheet: Toolkit for Teen Care, 2nd Edition

  • ACOG, Medical Student Education Module 2008

  • Comprehensive Gynecology Review, 3rd edition; editors, Ling FW, Vontver LA, Smith RP

  • Bloom and Fawcett: A Textbook of Histology, 11th edition, by Don W. Fawcett

  • Emans, Laufer, Goldstein's Pediatric and Adolescent Gynecology, 4th edition, by S. Jean Emans and Marc R. Laufer