Normal Menstrual Cycle

Normal Menstrual Cycle

The menstrual cyclemenstrual cycle is the series of changes in the body to prepare for a pregnancy. About once a month, the lining of the uterus (endometrium) starts to thicken. Then an ovary releases an egg. If the egg is fertilized by sperm and attaches to the lining of the uterus (implants), pregnancy begins. If the egg is not fertilized or a fertilized egg does not implant, the lining of the uterus is shed. This is the monthly menstrual bleedingmenstrual bleeding (also called menstruation or menstrual period) that happens from the early teen years until menopausemenopause, around age 50.

The menstrual cycle is from Day 1 of bleeding to Day 1 of the next time of bleeding. The average cycle is 28 days. But it's normal to have a cycle that is shorter or longer. For example:

• A teen's cycles may be long (up to 45 days), growing shorter over several years.
• Between ages 25 and 35, cycles are often regular. They generally last 21 to 35 days.
• Around ages 40 to 42, cycles tend to be the shortest and most regular. This is followed by 8 to 10 years of longer, less predictable cycles until menopause.

Menstrual periods usually start around age 12. But it may start earlier or later. It's common to have fewer periods between ages 39 and 51. If you are in your teens or in your 40s, you may have cycles that are longer or that change a lot. If you are a teen, your cycles should even out with time. If you are nearing menopause, your cycles will probably get longer and then will stop.

Phases of the menstrual cycle

The three phases of your menstrual cycle are triggered by hormonal changes.

Menstrual period

On day 1 of your cycle, the thickened lining (endometriumendometrium) of the uterus starts to shed. You know this as menstrual bleeding from the vagina. A menstrual period can last about 4 to 6 days.

Most of your menstrual blood loss happens during the first 3 days. This is also when you might have cramping pain in your pelvis, legs, and back. Cramps can range from mild to severe. The cramping is your uterus contracting, helping the endometrium shed. In most cases, any premenstrual symptomspremenstrual symptoms that you've felt before your period will go away during these first days of your cycle.

Follicular phase

During the follicular phase, an egg follicle on an ovary gets ready to release an egg (ovulationovulation). Usually, one egg is released each cycle. This process can be short or long. It plays the biggest role in how long your cycle is. At the same time, the endometrium starts to thicken to get ready for a fertilized egg.

The last 5 days of the follicular phase, plus ovulation day, are your fertile window. This is when you are most likely to become pregnant if you have sex without using birth controlbirth control.

Luteal (premenstrual) phase

This phase starts on ovulation day, the day the egg is released from the egg follicle on the ovary. It can happen anytime from day 7 to day 22 of a normal menstrual cycle. During ovulation, you may have less than a day of red spotting or lower pelvic pain or discomfort (mittelschmerz). These signs of ovulation are normal.

• If the egg is fertilized by sperm and then implants in (attaches to) the endometrium, a pregnancy begins. (This pregnancy is dated from day 1 of this menstrual cycle.)
• If the egg isn't fertilized or a fertilized egg doesn't implant, the endometrium starts to break down.

After the teen years and before perimenopauseperimenopause in your 40s, your luteal phase is very predictable. It normally lasts 13 to 15 days, from ovulation until menstrual bleeding starts a new cycle. This 2-week period is also called the "premenstrual" period.

It's common to have premenstrual symptoms during all or part of the luteal phase. You may feel irritable. You may gain water weight and feel bloated. Or you may have acne or tender breasts. A day or more before your period, you may start to have pain (cramps) in your belly, back, or legs. It's normal to have less energy at this time. You may also have headaches, diarrhea or constipation, nausea, or dizziness. When premenstrual symptoms make your daily life difficult, you are said to have premenstrual syndromepremenstrual syndrome (PMS).

Menarche and the Teenage Menstrual Cycle

Menarche (say "MEN-ar-kee") is your first menstrual period. Your period is a part of your menstrual cyclemenstrual cycle. This is a series of changes your body goes through to prepare for a possible pregnancy.