How your menstrual cycle works

What is a menstrual cycle?

You probably have a good idea about how your menstrual cycle works. But if you're hoping to get pregnant (or if you aren't), it's a great idea to make sure you understand the finer details of your cycle.

The menstrual cycle begins in puberty, when hormones help prepare the uterus for carrying a pregnancy each month. Over the course of roughly 28 days, the menstrual cycle includes a period, the maturing of eggs, and ovulation. The female reproductive systems include the uterus, ovaries, fallopian tubes, cervix, vagina, and breasts.

When you're born, your ovaries contain all your eggs – about 1 million of them. These small, oval shaped organs on each side of the uterus will release roughly 400 eggs during your fertile years, and your body will reabsorb those that aren't released.

At ovulation, ovaries release a mature egg into the fallopian tubes. Typically, this is where fertilization will occur if a sperm meets the egg at the right time. Then, the egg travels through the fallopian tubes to the uterus.

During pregnancy, your uterus holds your baby. As part of your menstrual cycle, the lining of your uterus thickens each month so it's hospitable if an egg is fertilized during ovulation. If sperm doesn't fertilize the egg, your body will release the egg and shed your uterine lining during your period.

What is a period?

Your period begins your menstrual cycle. This is when your body releases unfertilized eggs and sheds the lining of your uterus. Day 1 of your period is considered day 1 of your menstrual cycle.

Everyone's body is different, so a period can vary greatly from person to person. Typically, women begin having their period during puberty and continue this monthly cycle until they experience menopause in their late 40s to mid-50s.

Menstrual bleeding can last between 2 and 7 days, and is usually light to moderate, but can be heavy for some women.

Menstrual cycle phases

The average length of a menstrual cycle is 28 days, but anywhere from 21 days to 35 days is considered normal. This cycle is divided into phases based on how your hormones change over the course of a month:

Menstruation

Menstruation is the first phase of your cycle and typically lasts from day 1 to 5. When you get your period, your estrogen and progesterone levels are low. The first day of your menstrual period (when you begin to bleed) is called "cycle day one" – or "CD1."

Some women have regular periods, meaning their cycle always lasts the same number of days each month. Others find their cycle length varies – and that can be normal, too. But if your cycle length varies by more than a week for months at a time or if you're missing periods, it's a good idea to talk to your healthcare provider.

Follicular phase: around days 1 to 13

The first day of your period also marks the beginning of the follicular phase. During this phase, your body is preparing your uterus and your eggs for a possible pregnancy. Thanks to the Gonadotropin-releasing hormone (GnRH), which is produced in the hypothalamus region in your brain, levels of follicle-stimulating hormone (FSH) slowly rise.

FSH is produced in the pituitary gland, a small area near the hypothalamus. It tells the eggs in your ovaries to start "ripening" and controls the release of estrogen from the ovaries. Each egg is inside a sac called a follicle.

One follicle grows quicker than all the others. This dominant follicle is the one that will release an egg this cycle. In a 28-day cycle, the follicular phase typically lasts until about day 13. This phase accounts for most of the variation in women's cycle lengths: In a shorter cycle, the follicular phase is shorter; in a longer cycle, the follicular phase is longer.

FSH also stimulates the ovaries to produce estrogen. Estrogen encourages cells in the endometrium to grow. As a result, your uterine lining thickens and becomes spongier. Blood vessels also swell, increasing blood flow to the lining. These changes prepare your uterus to support a pregnancy. (If you don't get pregnant, this uterine lining is shed during your period.)

Estrogen also causes your cervical mucus to become thinner and more slippery. This type of mucus helps sperm cells slip more easily through the cervix and into the uterus.

Ovulation: around day 14

Ovulation – when an egg is released from the ovary – typically happens about 14 days before the first day of a woman's next period. So, in a 28-day cycle, ovulation may happen on cycle day 14. A rise in estrogen causes a surge of the luteinizing hormone (LH), which stimulates your ovaries to release eggs.

About 36 hours after an LH surge, the egg breaks out of the follicle. Almost immediately, the egg is swept into the fallopian tube by the finger-like projections that surround the tube's opening. There, the egg is in position to meet up with a sperm cell.

The egg survives in the fallopian tube for only about 12 to 24 hours. Sperm, however, can survive up to five days in your reproductive tract. So, if you ovulate on cycle day 15, for example, it's possible that sperm entering your body between cycle days 10 and 15 may reach your egg.

If you want to get pregnant, a good approach is to have sex two days before ovulation, so sperm are waiting in the fallopian tubes when the egg is released, and again on the day you ovulate. To improve your chances, experts often suggest having sex every other day around the time you expect to ovulate.

Luteal phase: around days 15 to 28

The luteal phase begins after you ovulate. In a 28-day cycle, it may start on day 15. Once this phase starts, levels of FSH and LH drop. The time for conception has passed, and your body is preparing for pregnancy – or your period.

In your ovary, the now-empty follicle collapses and becomes a small yellow mass of cells called the corpus luteum. The corpus luteum produces progesterone, which changes the mucus in the cervix. You may notice that your vaginal discharge becomes thicker and stickier during this stage of your cycle.

Progesterone also affects the lining of your uterus, which continues to thicken as a result of an increased blood supply. The lining secretes special substances that will nourish a fertilized egg.

If a sperm cell has successfully fertilized your egg, the developing ball of cells (called a zygote at first and then an embryo) makes its way down your fallopian tube toward your uterus. In about a week, it will likely implant in your uterine lining. At that point, you'll be pregnant!

Within a week or so of implantation, you may see a positive result on a home pregnancy test. And within another week or more you're likely to feel pregnancy symptoms. Often, one early clue that you're expecting is breast tenderness, which is caused by increased progesterone and estrogen. During pregnancy, levels of both hormones will skyrocket.

If the egg isn't fertilized or isn't viable, it degenerates as it travels along the fallopian tube to your uterus, and its microscopic remnants will leave your body along with your menstrual flow.

During the last few days of your cycle, if you're not pregnant, the levels of both progesterone and estrogen drop. This hormonal shift causes the blood vessels in the uterine lining to constrict, and without a steady blood supply, the uterine lining starts to break down.

Meanwhile, chemicals with hormone-like effects called prostaglandins– which are produced in the disintegrating uterine lining – make your uterine muscles contract and bring on menstrual cramps. Eventually, the blood vessels in the lining rupture, and the blood and tissue from your uterus flow out of your body through your vagina. In other words, you get your period.

Then the cycle starts again. Except during pregnancies, your body will likely continue this incredible process until menopause.

What things affect my menstrual cycle?

Some variation in your menstrual cycle is normal. Your cycle could be anywhere from 21 days to 35 days, and the individual phases of your cycle could vary by a few days, too. However, if your cycle is markedly irregular, health or environmental factors could be affecting your menstrual cycle. Examples of an irregular cycle include going 45 days or more between periods, missing periods, having increased pain during periods, or having heavy bleeding lasting more than 7 days.

Some of the most common things that can affect your cycle include:

• Pregnancy and breastfeeding
• Significant weight loss along with over-exercising or disordered eating.
• Excessive cortisol, the hormone produced by the body when under stress
• Medical conditions including polycystic ovary syndrome, premature ovarian failure, pelvic inflammatory disease, and uterine fibroids.

Should I be concerned about missing periods?

If you missed a period, and confirmed you're not pregnant, there might not be any reason to worry. The occasional missed period typically isn't reason for concern. However, it's a good idea to take note of missed periods and other period irregularities. If these symptoms continue, there may be something going on with your hormones.

If you miss more than one period, talk with your doctor. They can help you figure out what's going on and how to get your cycle back on track.