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You probably have a good idea of what happens during your menstrual cycle. Your body prepares for pregnancy and releases an egg. If the egg isn't fertilized by a sperm cell, your body sheds it, as well as your uterine lining, during your period. Then the cycle repeats.
Those are the basics. But if you're hoping to get pregnant or just want to understand your body better, it's important to know some of the finer details of how this complex, hormone-driven process works.
Your reproductive system
The female reproductive system is made up of the:
- Uterus: Holds your baby during pregnancy. The lining of the uterus – called the endometrium – thickens every month in case it's needed to support a fertilized egg. Amazing fact: Before pregnancy, your uterus is only about the size of a small orange. By the end of pregnancy, it's about 5 times that size.
- Ovaries: Two small, almond-shaped organs – one on each side of the uterus – that contain your eggs. You're born with 1 million to 2 million eggs, and you'll probably release about 400 during your fertile years. Most of the others get reabsorbed into your body, so when you reach menopause – at age 51, on average – you'll have about 1,000 eggs remaining.
- Fallopian tubes: Two tubes (one on each side, for each ovary) that "catch" the egg after it's released from the ovary and transport it to the uterus.
- Cervix: The opening of the uterus into the vagina.
- Vagina: The passageway from the uterus to the outside of your body.
Cycle days and length
The average length of the menstrual cycle is 28 days, but anywhere from 23 days to 32 days is considered normal. The first day of your menstrual period (when you begin to bleed) is called "cycle day one" – or "CD1." (Here's a guide to decoding other fertility terms and abbreviations.)
Some women have regular periods, meaning their cycle always lasts the same number of days. 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.
Your reproductive hormones
Hormones are chemical messengers that travel through the bloodstream to an organ or tissue. Once the hormone "locks on" to the cells of that organ or tissue, it sends a chemical signal that tells the cells what to do.
In women, the main reproductive hormones are:
- Gonadotropin-releasing hormone (GnRH), which is produced in the hypothalamus region in your brain. GnRH stimulates the release of two additional hormones that are crucial for reproduction: follicle-stimulating hormone and luteinizing hormone.
- Follicle-stimulating hormone (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.
- Luteinizing hormone (LH) is also produced in your pituitary gland. It stimulates your ovaries to release eggs.
- Estrogen is produced in your ovaries and has many roles in your body, from causing your breasts to develop to maintaining the tissues of your reproductive tract.
- Progesterone, also produced in your ovaries, works with estrogen to keep your reproductive cycle regular and maintain pregnancy.
Menstrual cycle: Period, follicular phase, ovulation, luteal phase
Your menstrual cycle can be divided into several phases: menstruation (your period), the follicular phase, ovulation, and the luteal phase. Here's what happens during each one. (The timing shown here assumes a typical 28-day cycle.)
Menstruation: Days 1 to 5
When you get your period, your estrogen and progesterone levels are low. Menstrual bleeding lasts three to seven days, or about five days on average.
Follicular phase: 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 hormone GnRH, levels of FSH slowly rise and spur your ovaries to start readying 15 to 20 eggs. Each egg is encased in a sac called a follicle.
One follicle grows quicker than all the others, reaching a diameter of 18 millimeters to 25 millimeters (about 1 inch). This dominant follicle is the one that will release an egg this cycle. (If your body produces two dominant follicles and both release eggs that are fertilized, they become fraternal twins.)
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, which has several effects on your reproductive system at this stage of your cycle.
- Estrogen stimulates 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 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: 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.
What triggers ovulation? A rise in estrogen, which causes a surge of LH from the pituitary gland. (You can buy an ovulation predictor kit to help you identify when this is happening, so you have advance notice that you're about to ovulate. If you don't want to buy a kit, you can try charting your cycle or using the calendar method to predict ovulation.)
About 36 hours after an LH surge, the egg breaks out of the follicle. (The empty follicle stays behind and plays an interesting role a bit later in your cycle.)
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: 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 hormone 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, inflammatory pain chemicals 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.