This may come as a surprise, but your 'monthly' cycle doesn’t necessarily take place once a month. The average 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, it’s likely that you will have a period more often than once a month. However, if your cycle lasts longer, you’re one of the girls who will have fewer periods in a year.
Most girls get their first period between 11 and 13 years old, though it can start anywhere from the age of eight to 16 years old. Generally, a woman has about 480 periods (fewer if she has pregnancies) throughout her adolescence and adulthood until she reaches menopause (often around the age of 51), at which point, her periods stop.

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

• Two ovaries: this is where eggs (ova) are stored and released. A human egg is tiny (120 microns or micrometres). This is about the width of a human hair.
• The womb (uterus): where a fertilised egg implants and a pregnancy grows
• Two fallopian tubes: these are two thin tubes that connect the ovaries to the womb
• The cervix: the lower part of the womb that connects to the vagina
• The vagina: a tube of muscle connecting the cervix to the outside of the body. (Your vagina is actually inside your body – the part on the outside that you can see and which is frequently mistakenly called the vagina, is your vulva.)

Each month, your reproductive system repeats a regular pattern of events that are controlled by hormones.
Hormones are substances produced by your body that control your body’s functions. As you approach puberty, a part of your brain called the pituitary gland begins releasing more and more of some specific hormones.
These hormones stimulate your ovaries to produce oestrogen and another part of your body to produce other hormones called androgens. These hormones cause many of the physical changes that take place during puberty and over the phases of your monthly cycle.

1. Pre-ovulation (called the follicular phase)
   Women and girls have two ovaries that contain thousands of eggs (ova). During this phase, hormones stimulate the development of eggs; 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. If sperm from a male fertilises the egg, a baby develops. Ovulation usually happens around 10 to 16 days before the next period.
3. Premenstrual (called the luteal phase)
   After ovulation, hormones trigger your body to continue developing the lining of your uterus, in preparation for a fertilised egg. During this phase, if you become pregnant, the egg moves into your uterus and then attaches to the lining. If you are not pregnant, the lining of the uterus is shed through the vaginal opening.
4. Menstruation
   The womb lining leaves your body through the vagina as a reddish fluid containing blood – typically about a quarter of a cup of blood (though it can seem like a lot more). This is your period – it is also called menses – and it will last between three to seven days. The first day of bleeding is day one of your period.

• Over the course of a lifetime, you release about 400 eggs in their mature form
• The number of eggs that are contained in the ovaries depends on how old you are
• The highest number of eggs are released before you are born
• As a 20-week-old female foetus in your mother’s uterus, you have approximately seven million eggs
• At birth, the number drops to two million
• By the time you start puberty, you have between 300,000 and 500,000 eggs
• This decline in number is called atresia, a natural and continuous process
• Only between 400 and 500 will ripen into mature eggs during a lifetime