Menstrual Cycles

What is your LMP? This is a standard question on health history forms, more often asked at the Ob/Gyn office than others. The LMP is the first day of the Last Menstrual Period. The LMP is also called day one of the menstrual cycle, or CD 1 (cycle day 1).

In order to accurately answer this question, the first day of the LMP is defined as the first day that there is enough vaginal bleeding to require protection. This means that a tiny amount of spotting or staining technically does not count as CD 1, only when there is enough blood to warrant the use of a pad or tampon. The last day of the cycle is the day before the next CD 1 appears.

Most (but not all) menstrual cycles are 28 days long. For purposes of discussion we can describe the events that occur during a “textbook” 28-day menstrual cycle.

There are four phases in a menstrual cycle:

  • Menstruation
  • Follicular
  • Ovulatory
  • Luteal

Each phase can vary in length and each has specific biological significance.

The purpose of the menstrual cycle is to enable pregnancy; keeping this in mind will help you to understand the four phases.

The Menstruation Phase (the period)

Days 1–5: This is the start of a new menstrual cycle. There is vaginal bleeding, usually lasting four to five days, and often the first day or two are perceived as “heavy.” This bleeding is the shedding of the uterine lining because pregnancy did not occur. A normal period can last up to seven days, but for this discussion we will assume five. Periods longer than seven days are therefore abnormal, but not necessarily a sign of a problem. Regardless of whether a woman starts her period on the first of the month or the seventeenth of the month, the day she begins bleeding is considered the first day of her menstrual cycle (CD 1).

Every month, the uterus builds a new lining of blood and tissue in order to nourish a developing baby if a woman is pregnant. If she is not pregnant and this lining (called the endometrium) is not needed, it sheds and is expelled from the uterus, resulting in menstruation.

The start of a woman’s menstruation is a key sign that she has not become pregnant. If a woman is pregnant, however, she may encounter light bleeding or spotting caused by implantation. Therefore, the appearance of bleeding does not always mean that the woman is not pregnant.

The start of the period is often marked by abdominal or pelvic cramping, bloating, backaches, fatigue, and symptoms of PMS (premenstrual syndrome), but once menstruation begins and the blood begins flowing, these symptoms tend to subside within the first couple of days. Sanitary napkins, pads, and tampons are generally worn during this phase to prevent the menstrual blood from spreading or transferring to other surfaces. An enzyme called plasmin prevents clotting of the menstrual fluid. A woman can develop an iron deficiency during this time because of the blood loss (usually 10–80 ml throughout the phase), which occurs month after month, potentially depleting the body of stored iron.

Not all bleeding during a menstrual cycle is menstruation, especially if there is bleeding before or after days 1–5. During this phase, the pituitary gland releases a hormone called FSH (follicle stimulating hormone), which stimulates the ovaries to make eggs.

The Follicular Phase

Days 5–13. This is when the eggs are developing inside their follicles. Many eggs begin this process, but normally only one egg matures enough to be released, and there is only one egg per follicle. Sometimes two follicles develop: This can result in two eggs being released, which can lead to a twin pregnancy. Twins caused by two eggs being fertilized are fraternal twins, and can never be identical. Fraternal twins are no more alike than any two siblings would be.

Early in the menstrual cycle, the pituitary gland is stimulated to start production of FSH (follicle stimulating hormone). FSH stimulates the ovaries to start a round of follicle development. A small amount of LH (luteinizing hormone) is also released, but there is much more FSH.

The trigger to release FSH (and LH) actually comes from a tiny gland in the brain called the hypothalamus. It releases a hormone called GnRH (gonadotropin releasing hormone), which causes the pituitary to release FSH and LH.

The follicles start to make the eggs, and they also begin to produce estrogen hormone (estradiol mainly), which is released directly into the bloodstream. Estradiol causes the uterine lining to grow thicker and thicker. This “follicular” phase of the cycle is also called the proliferative phase, because the cells of the uterine lining are multiplying (proliferating). The lining has to become thick enough to sustain a pregnancy if one occurs.

Estradiol production rises, and soon it works to reduce the production of FSH. This is called a negative feedback loop. The loss of FSH causes most of the eggs to regress, with one main follicle becoming the “winner.” Also, once a follicle is mature, it will continue to grow and grow, producing more and more estrogen, even if the FSH were to disappear.

The next step is one of Mother Nature’s small miracles. At some point the high levels of estradiol, or perhaps the rate of change (nobody knows for sure) triggers a sudden surge in the amount of LH released by the pituitary. Likely the high estradiol is stimulating more GnRH. This LH surge can be measured and is the basis for most home urine ovulation prediction tests. LH converts the mature follicle into an egg sac that is ready to ovulate, which usually occurs 12–36 hours later (see ovulation phase).

The Ovulation Phase (mid cycle)

The LH surge doesn’t last very long—usually 24–36 hours. Once the surge is done and over with, the menstrual cycle moves out of the follicular phase and into the ovulation phase.

Day 14: This is the date of ovulation (in a 28-day cycle). As ovulation approaches, the blood supply to the ovary increases and the ligaments contract, pulling the ovary closer to the fallopian tube, allowing the egg, once released, to find its way into the tube. Just before ovulation, a woman’s cervix secretes a lot of clear “fertile mucous,” which is characteristically stretchy.

When ovulation occurs, the egg leaves the follicle and enters the fallopian tube. Inside the body, the follicle opens and spills its fluid. This is called follicle rupture. The follicle does not burst, as most people think. It weakens enough for a hole or tear to develop, and this releases the egg sac fluid and the egg. Inside the fallopian tube, the egg is carried along by tiny projections called “cilia” toward the uterus. Fertilization occurs if sperm are present as the live egg reaches the uterus.

If a woman is trying to become pregnant, this is her most “fertile” day. Some women use daily mucous monitoring to determine when they are most likely to become pregnant. At this middle stage of the menstrual cycle, some women may also experience spotting, cramping or other sensations. The cervical mucus can become clear and watery enough to require some type of liner for protection. The two or three days prior to ovulation are also good “fertile” days. A woman’s libido is often higher mid-cycle because Mother Nature has designed the process to lead to pregnancy.

Ovulation can be delayed by stress, illness, medication, or extremely heavy and prolonged physical activity or exercise. Women often say things like “my period is late,” but it’s actually the ovulation phase of the cycle that is late. This can be confusing if a woman is not charting her cycles.

The Luteal Phase

LH has one more job to do. Under its influence, after ovulation, the now empty follicle develops into the corpus luteum gland. This gland, which only lives for a short while, secretes progesterone, defining the luteal phase (see below).

Days: 15–28: The follicle has released the egg, becomes the corpus luteum, and now starts to produce the hormone progesterone. This hormone changes the uterine lining to make it ready for the implantation of a fertilized egg. The lining becomes thick and soft; imagine the egg sinking into a down comforter.

Progesterone also increases body temperature, which is why women used to take their temperature when trying to become pregnant. A spike in body temp of about 0.5 degrees often occurs about 1-2 days after ovulation. In the absence of pregnancy, progesterone is produced for about 12 days or so, so a drop in body temp can indicate that pregnancy did not occur, and this will be followed in 1-3 days by the next menstrual period. A sustained elevated temp beyond the expected 12 days might be first indication that a pregnancy has occurred.

It takes about three days for a fertilized egg to move through the fallopian tube, which gives the body enough time for the lining to change to accept the fertilized egg. The luteal phase is also called the columnar or glandular phase, due to the changes in the uterine lining (under the microscope the uterine lining is very swollen and is rich in glands).

During the last few days of this cycle, the drop in progesterone can trigger symptoms such as bloating, cramping, mood changes, skin changes, and more. This is commonly referred to as PMS (premenstrual syndrome).


The first hormone to be detected when pregnancy occurs is hCG. This is called human chorionic gonadotropin. The chorion is the brand new placenta being formed. The first job of hCG is to force the corpus luteum to keep producing progesterone. This is why the body temperature stays elevated after day 14 of the luteal phase. This is also why many women develop cysts early in pregnancy (often called corpus luteum cysts); because hCG is so strong, it can cause the corpus luteum to enlarge and become a cyst.

Luteal Phase Defect

The luteal phase occurs during the last two weeks of a woman’s menstrual cycle. Luteal phase defect occurs when the ovary (the corpus luteum) does not produce a normal amount of progesterone. Insufficient production of progesterone in amount or duration can lead to abnormal development of the luteal phase endometrium and can cause a luteal phase that is too short, not of good quality, or both, which in turn can have a negative effect on a woman’s fertility.

Cause and Effect

The number one reason for the luteal phase defect is low progesterone production. Poor follicle production happens when a woman doesn’t produce the correct amount of FSH during the follicular phase. While a woman doesn’t produce FSH, she doesn’t develop the follicles that later become the corpus luteum, and the quality of the corpus luteum is reduced dramatically, along with the levels of progesterone. The uterine lining doesn’t respond to follicle development, leaving the lining unprepared for implantation. If the uterine lining isn’t prepared, there’s no way for the egg to attach itself to it, and pregnancy is impossible.

Diagnosing Luteal Phase Defect

Luteal phase defect can be diagnosed by performing a biopsy of the endometrium after the twenty-first day of a woman’s menstrual cycle (ideally about day 25–26). CD 25–26 is used so that the lab can determine if the microscopic findings match the expected development based how many days post-ovulation the tissue is obtained. If there is more than a two-day discrepancy, the lining is said to be “out-of-phase.”

Alternatively, a progesterone blood test can be done on the woman’s blood at about cycle day 21. Cycle Day 21 is used for the blood test because this is the peak of the luteal phase; this is also called the mid-luteal point.

The second method is preferable due to the difficulty of precise dating of the menstrual cycle, the cost of endometrial biopsy, and the painful nature of the procedure that is required to obtain tissue samples for biopsy.

Both these tests require precise knowledge of the date of ovulation, so either an OPK (ovulation predictor test) is used or the women has been doing BBT (basal body temperature charting).

If you have been having difficulty conceiving—especially if you have a history of repeated miscarriages—then there is a possibility that luteal phase defect may be the problem. Even if you have not been experiencing irregular periods; it is possible for hormonal abnormalities to occur without disturbing the menstrual cycle in any significant way. Up to 30 percent of women may suffer from luteal phase defects.

Ask yourself the following questions to determine whether you should see a doctor to determine whether you have a luteal phase defect:

  • Is your luteal phase shorter than twelve days? (Fourteen days is normal.)
  • After ovulation, does your basal body temperature rise slowly, or not very much?
  • Do you experience excessive spotting before your period?

Treatment of Luteal Phase Defect

Luteal phase defect is sometimes treated by administering supplemental progesterone, either orally, by injection, or via vaginal suppositories or gel. While not many studies have been done to evaluate the efficacy of this kind of treatment, success rates have been reported to be around 50 percent. Another method used in some cases is to administer hCG hormone supplements to encourage the corpus luteum to produce more progesterone. This is normally done at a fertility clinic.

The Next Cycle

If the cycle is 28 days and pregnancy does not take place, then bleeding again occurs on day 29. Thus Day 29 becomes Day 1 of the next cycle. When fertilization does not occur, the follicle stops producing progesterone after about 12 days. The loss of progesterone causes the thickened uterine lining to shed, which causes vaginal bleeding, and this begins the next menstrual cycle.

Menstrual Cycle Myths

  • “Every woman’s cycle is 28 days.” This is true for most, but not all women follow a “regular” 28-day cycle. Some women have a 26-day cycle, some a 35-day cycle. Some women have 28-day cycles every month, and some have cycles that vary in length from month to month.
  • “Every woman will or should bleed monthly.” This too is untrue. Not all women bleed every month, for various reasons. Some women have a period every six weeks. Going more than two or three months without a period (if you are not pregnant) warrants a medical evaluation.
  • “Every woman will or should ovulate monthly.” Not all women ovulate monthly, but a woman does need to ovulate periodically. Some women produce hormones and do not ovulate. This can lead to problems if untreated. Some women ovulate but not monthly—maybe every two to three months. This is okay unless you are attempting to get pregnant, in which case you should see an ob/gyn. Also, if you are charting your ovulation cycle and not ovulating, or the urine ovulation predictor test does not show ovulation, it is a good idea to see an ob/gyn doctor or a fertility specialist.
  • “If a woman bleeds, she’s not pregnant.” It is not unheard of for a woman to continue to bleed throughout the first trimester of pregnancy. Look for other pregnancy symptoms and signs to find out if you’re pregnant.
  • “A woman cannot get pregnant while menstruating.” Technically this is true, but many women have light spotting at the time of ovulation and they may think they are having another period, so they decide that they do not need birth control. All we can say about this is Oops!

This page was last updated on 06/2017

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