Here's what happens:
The uterus is the place inside a woman's body where a fetus (baby) develops. Every month or so, the lining of the uterus gets thicker to prepare for a fertilized egg if the woman becomes pregnant. If the egg doesn't get fertilized, that lining is released from the body through the vagina. This is your period - or "menstruation."
Why learn about this stuff?
If you're a girl, getting to know this part of your body will help you understand one of puberty's biggest events: your first period. This is useful for knowing when to carry feminine protection, predicting the symptoms you might have during your cycle, and later, understanding your reproductive system in the event you should want to use birth control or become pregnant.
Phases of the Cycle:
The following is a breakdown of the four phases of a menstrual cycle.
Phase One-
Menstruation - "Having your period"
• What's happening: Your body is getting rid of tissue it doesn't need.
• More information: When you menstruate, the lining of the uterus (the endometrium) breaks up and passes slowly from the uterus through the vagina to the outside of your body. This menstrual "flow" drips out slowly. It can look like a lot of blood (it's only about 35 ml blood - some women lose more), but it's actually a mixture of blood, mucus, and body cells amounting to a total of about 6-9 tablespoons (or about 80-85 ml) of menstrual fluid on average for each period. The flow might be red or quite dark, and might include some clumps or clots.
Phase Two -
Pre-Ovulation Phase (Right after your period ends)
• What's happening: Your body is preparing an egg for pregnancy.
• More information: About the time your flow stops, your ovaries start to prepare another egg, or ovum, for release into one of the fallopian tubes (usually, the ovaries take turns releasing eggs - the left ovary one month, the right ovary the next month). At the same time, a hormone called estrogen tells your uterus to build up the endometrium.
Phase Three -
Ovulation
• What's happening: A lot happens in this phase, but basically the egg is released from the ovary into the fallopian tubes, and your body is preparing the uterus to receive a fertilized egg (if there is one).
• More information: As Phase Two ends, your brain sends a new hormonal signal to your ovaries, telling one ovary to release the mature egg (ovum). This step is called ovulation. First, the egg moves through the fallopian tube toward the uterus. At the same time, the endometrium is growing even thicker because of the hormone progesterone. This happens so the body is ready in case the egg is fertilized. If the egg isn't fertilized, your body knows that it doesn't need its "nest" any longer.
Although menstrual cycles can vary in length, the number of days between ovulation and the menstrual period is fairly consistent - ovulation usually occurs 14 days before the start of your next period.
Phase Four -
The Premenstrual (Luteal) Phase
• What's happening: if a woman has not become pregnant, the uterus gets ready to release the unneeded tissue that was prepared for the egg.
• More information: Levels of two hormones involved in the development of the uterine lining, estrogen and progesterone, begin to drop. The stimulation for the endometrium is lost. This causes the shedding of the lining to begin, and a new menstrual cycle starts.
When will I get it?
Menstruation will begin sometime during puberty. The average age is 12 or 13, but anywhere between 10 and 16 is still normal. There is no "right" time - your period will begin when your body is ready. If you haven't started by the time you are 16 or 17, you should see your doctor.
How long will my period/cycle last?
When you first start menstruating, the length of your period won't be regular -- it could last one day or 10 days. The average length of a period is 3-7 days.
When you first start to menstruate, the cycle could be very irregular; starting, stopping and starting again. For example, you could have one period and then wait as long as six months for the next one! This isn't unusual. Until your body adjusts to your cycle, your period may be unpredictable. But after that, your menstrual cycle should be fairly regular during most of your menstruating years. The length of time between periods can be anywhere from 21-35 days. If your period continues to be very irregular, you should see your doctor.
Figuring out your cycle:
Although it can take two years or more before you notice a regular pattern, it's a good idea to get to know your cycle right from the start (Use a period calendar). On your calendar each month, circle or shade in the days that you menstruate. As your cycle evens out, you'll begin to see a regular pattern on your calendar. Then you can see when your next period will probably start by figuring out the average number of days in between your periods. You can also record how you feel on those days before your period starts. Then you can know your body's "hints" that your period is about to come.
Good To Know:
The word "menstruation" comes from menses, the Latin word for "month." That gives you a big clue to what it's all about. Or if you've heard friends talk about "that time of the month," you can probably guess that women menstruate about every month.
A menstrual cycle is the time between the first day of one period and the first day of your next period. The amount of time it takes for this to happen is about a month (the average cycle lasts 28 days). One complete cycle is made up of several phases.
The average woman has approximately 500 periods in her lifetime. They stop when a woman is about 50 years old (this is called the menopause).
When a baby girl is born, she has approximately 200,000 eggs in each of her ovaries (she has two ovaries). For your perspective it is medically not correct to say if you have 500 periods - 500 eggs will ripen because not in every menstrual cycle a egg ripens there are also un-ovulatory cycles especially in young girls and woman who are peri-menopausal (close to menopause). The first menstrual cycles tend to be an-ovular (no egg ripens). By 6 years after menarche occured 80% of the menstrual cycles are ovulatory and over 95% of the cycles are ovulatory by 12 years after the on-set of menarche.