The menstrual cycle is the regular natural change that occurs in the female reproductive system (specifically the uterus and ovaries) that makes pregnancy possible. The cycle is required for the production of oocytes, and for the preparation of the uterus for pregnancy.[rx] The menstrual cycle occurs due to the rise and fall of estrogen.[rx] This cycle results in the thickening of the lining of the uterus, and the growth of an egg, (which is required for pregnancy).[rx] The egg is released from an ovary around day fourteen in the cycle; the thickened lining of the uterus provides nutrients to an embryo after implantation.[rx] If pregnancy does not occur, the lining is released in what is known as menstruation or a “period”.[rx]
The reproductive system of a female, unlike men, shows regular cyclic changes that teleologically may be regarded as periodic preparation for pregnancy and fertilization. In primates and humans, the cycle is a menstrual cycle, and its most conspicuous feature is the periodic vaginal bleeding that occurs with the shedding of uterine mucose (menstruation). The length of the cycle is notoriously variable, but an average figure is 28 days from the start of one menstrual period to the start of next. By common usage, the days of the cycle are identified by number starting with the first days of menstruation. It begins at puberty, ranging from the ages of 10 to 16, and ends at menopause at an average age of 51.[1][rx][rx]
Function
Hormones are secreted in a negative and positive feedback manner to control the menstrual cycle. Hormone secretion begins in the hypothalamus where gonadotropin-releasing hormone (GnRH) is secreted in an increased, pulsatile fashion once puberty starts. GnRH is then transported to the anterior pituitary, where it activates its 7-transmembrane G-protein receptor. This provides a signal to the anterior pituitary to secrete stimulating follicle hormone (FSH) and luteinizing hormone (LH). FSH and LH provide input to the ovaries. Within the ovarian follicle, there are 2 cell types responsible for hormone production, theca cells and granulosa cells. LH stimulates theca cells to produce progesterone and androstenedione by activating the enzyme, cholesterol desmolase. Once androstenedione is secreted, the hormone diffuses to the nearby granulosa cells. Here, FSH stimulates the granulosa cells to convert androstenedione to testosterone then 17-beta-estradiol by activating the enzyme, aromatase. As levels of 17-beta-estradiol or progesterone increase based on the phases of the menstrual cycle, there is negative feedback back to the anterior pituitary to lower the levels of FSH and LH being produced and subsequently, the levels of 17-beta-estradiol and progesterone produced. An exception to this is during ovulation. In this case, once a critical amount of 17-beta-estradiol is produced, it provides positive feedback to the anterior pituitary to produce increased amounts of FSH and LH. This feedback system is represented in figure 1. Additionally, within the feedback system, the granulosa cells produce inhibin and activin, which inhibit and stimulate FSH release from the anterior pituitary, respectively. This feedback mechanism is controlled by upregulating, to increase hormone production, or downregulating to decrease hormone production, the GnRH receptors on the anterior pituitary.[rx][rx][rx]
Mechanism
Phase 1: The Follicular, or Proliferative Phase
The first phase of the menstrual cycle is the follicular or proliferative phase. It occurs from day zero to day 14 of the menstrual cycle, based on the average duration of 28 days. The variability in length of the menstrual cycle occurs due to variations in the length of the follicular phase. The main hormone during this phase is estrogen, specifically 17-beta-estradiol. The increase in this hormone occurs by the upregulation of the FSH receptors within the follicle at the beginning of the cycle. However, as the follicular phase progresses to the end, the increased amounts of 17-beta-estradiol will provide negative feedback to the anterior pituitary. The purpose of this phase is to grow the endometrial layer of the uterus. 17-beta-estradiol achieves this by increasing the growth of the endometrial layer of the uterus, stimulating increased amounts of stroma and glands, and increasing the depth of the arteries that supply the endometrium, the spiral arteries.
Additionally, this phase is also essential to create an environment that is friendly and helpful to possible incoming sperm. 17-beta-estradiol achieves this by creating channels within the cervix, allowing for sperm entry.[7] The channels are made within the abundant, watery, and elasticity changes of the cervical mucous. During this phase, a primordial follicle begins to mature into a Graafian follicle. The surrounding follicles begin to degenerate, which is when the Graafian follicle becomes the mature follicle. This sets up the follicle for ovulation, the next step.
Ovulation
Ovulation always occurs 14 days before menses; therefore, with an average 28-day cycle, ovulation occurs on day 14. At the end of the proliferative phase, 17-beta-estradiol levels are at a high due to the follicle maturation and increased production of the hormone. During this time only, 17-beta-estradiol provides positive feedback for FSH and LH production. This occurs when a critical level of 17-beta-estradiol is reached, at least 200 picograms per milliliter of plasma. The high levels of FSH and LH present during this time is called the LH surge. As a result, the mature follicle breaks, and an oocyte is released. The changes to the cervix as initiated during the follicular phase further increase, allowing for increased, waterier cervical mucous to better accommodate the possible sperm—the levels of 17-beta-estradiol fall at the end of ovulation.
Phase 2: The Luteal or Secretory Phase
The next phase of the menstrual cycle is the luteal or secretory phase. This phase always occurs from day 14 to day 28 of the cycle. Progesterone stimulated by LH is the dominant hormone during this phase to prepare the corpus luteum and the endometrium for possible fertilized ovum implantation. As the luteal phase ends, progesterone will provide negative feedback to the anterior pituitary to decrease FSH and LH levels and, subsequently, the 17-beta-estradiol and progesterone levels. The corpus luteum is a structure formed in the ovary at the site of the mature follicle rupture to produce 17-beta-estradiol and progesterone, which is predominant at the end of the phase due to the negative feedback system. The endometrium prepares by increasing its vascular supply and stimulating more mucous secretions. This is achieved by the progesterone stimulating the endometrium to slow down endometrial proliferation, decrease lining thickness, develop more complex glands, accumulate energy sources in the form of glycogen, and provide more surface area within the spiral arteries.
Contrary to the cervical mucous changes seen during the proliferative phase and ovulation, progesterone decreases and thickens the cervical mucous making it non-elastic since the fertilization period passed, and sperm entry is no longer a priority. Additionally, progesterone increases the hypothalamic temperature, so body temperature increases during the luteal phase. Near the end of the secretory phase, plasma levels of 17-beta-estradiol and progesterone are produced by the corpus luteum. If pregnancy occurs, a fertilized ovum is implanted within the endometrium, and the corpus luteum will persist and maintain the hormone levels. However, if no fertilized ovum is implanted, then the corpus luteum regresses, and the serum levels of 17-beta-estradiol and progesterone decrease rapidly.
Normal Menstruation
When the hormone levels decrease, the endometrium layer, as it has been changed throughout the menstrual cycle, is not able to be maintained. This is called menses, considered day 0 to day 5 of the next menstrual cycle. The duration of menses is variable. Menstrual blood is chiefly arterial, with only 25% of the blood being the venous blood. It contains prostaglandins, tissue debris, and relatively large amounts of fibrinolysis from endometrial tissue. The fibrinolysis lyses clot so that menstrual blood does not contain clots typically unless the flow is heavy.
The usual duration of the menstrual flow is 3-5 days, but flows as shorts as 1 day and as long as 8 days can occur in a normal female. The amount of blood loss can range from slight spotting to 80 mL and the average being 30 mL. Loss of more than 80 mL of the blood is considered abnormal. Various factors can affect the amount of blood flow, including medications, the thickness of the endometrium, blood disorders, and disorders of blood clotting, etc.
A general overview of the menstrual cycle
The menstrual cycle includes several phases. The exact timing of the phases of the cycle is a little bit different for every woman and can change over time.
| Cycle days (approximate) | Events of the menstrual cycle |
| Days 1-5 | The first day of menstrual bleeding is considered Day 1 of the cycle.
Your period can last anywhere from 3 to 8 days, but 5 days is average. Bleeding is usually heaviest on the first 2 days. |
| Days 6-14 | Once the bleeding stops, the uterine lining (also called the endometrium) begins to prepare for the possibility of a pregnancy.
The uterine lining becomes thicker and enriched in blood and nutrients. |
| Day 14-25 | Somewhere around day 14, an egg is released from one of the ovaries and begins its journey down the fallopian tubes to the uterus.
If sperm are present in the fallopian tube at this time, fertilization can occur. In this case the fertilized egg will travel to the uterus and attempt to implant in the uterine wall. |
| Days 25-28 | If the egg was not fertilized or implantation does not occur, hormonal changes signal the uterus to prepare to shed its lining, and the egg breaks down and is shed along with lining.
The cycle begins again on Day 1 menstrual bleeding. |
A comprehensive explanation of the menstrual cycle
The menstrual cycle has three phases:
1. Follicular Phase (Days 1-14)
This phase of the menstrual cycle occurs from approximately day 1-14. Day 1 is the first day of bright red bleeding, and the end of this phase is marked by ovulation. While menstrual bleeding does happen in the early part of this phase, the ovaries are simultaneously preparing to ovulate again. The pituitary gland (located at the base of the brain) releases a hormone called FSH – follicle-stimulating hormone. This hormone causes several ‘follicles’ to rise on the surface of the ovary. These fluid-filled “bumps” each contain an egg. Eventually, one of these follicles becomes dominant and within it develops a single mature egg; the other follicles shrink back. If more than one follicle reaches maturity, this can lead to twins or more. The maturing follicle produces the hormone estrogen, which increases over the follicular phase and peaks in a day or two prior to ovulation. The lining of the uterus (endometrium) becomes thicker and more enriched with blood in the second part of this phase (after menstruation is over), in response to increasing levels of estrogen. High levels of estrogen stimulate the production of gonadotropin-releasing hormone (GnRH), which in turn stimulates the pituitary gland to secrete luteinizing hormone (LH). On about day 12, surges in LH and FSH cause the egg to be released from the follicle. The surge in LH also causes a brief surge in testosterone, which increases sex drive, right at the most fertile time of the cycle.
2. Ovulatory Phase (Day 14)
The release of the mature egg happens on about day 14 as a result of a surge in LH and FSH over the previous day. After release, the egg enters the fallopian tube where fertilization may take place, if sperm are present. If the egg is not fertilized, it disintegrates after about 24 hours. Once the egg is released, the follicle seals over and this is called the corpus luteum.
3. Luteal Phase (Days 14-28)
After the release of the egg, levels of FSH and LH decrease. The corpus luteum produces progesterone. If fertilization has occurred, the corpus luteum continues to produce progesterone which prevents the endometrial lining from being shed. If fertilization has not occurred, the corpus luteum disintegrates, which causes progesterone levels to drop and signals the endometrial lining to begin shedding.
What is normal bleeding?
There is a range of normal bleeding – some women have short, light periods and others have longer, heavy periods. Your period may also change over time.
Normal menstrual bleeding has the following features:
- Your period lasts for 3-8 days
- Your period comes again every 21-35 days (measured from the first day of one period to the first day of the next)
- The total blood loss over the course of the period is around 2-3 tablespoons but secretions of other fluids can make it seem more
How can I figure out what is happening in my cycle? When am I ovulating?
Simply tracking your cycle on a calendar, along with some details of your bleeding and symptoms can help you understand your cycle. Record when your period starts and ends, what the flow was like, and describe any pain or other symptoms (bloating, breast pain, etc.), changes in mood, or behavior that you experienced. Over several cycles, you will be able to see patterns in your cycle or identify irregularities that are occurring. Use your own calendar or try this ‘menstrual diary’. There are also numerous apps available to help you track your period. If your periods come regularly every 21-35 days, chances are excellent that you are ovulating.
Beyond simple calendar tracking, there are a few ways to figure out the timing of your own personal menstrual cycle. Separately or used together, these can be used to help determine when and whether you are ovulating. Three methods you can try are cervical mucus testing, basal body temperature monitoring, and ovulation prediction kits.
Cervical mucus testing
What is cervical mucus?
The cells lining your cervical canal secrete mucus. The consistency of this mucus changes over your cycle. When you are fertile, the mucus changes to consistency and structure that permits the sperm’s travel on its way to your egg. When you are most fertile it will be clear, abundant, and stretchy. To give you an idea of the consistency, this type of fertile mucus is sometimes abbreviated as EWCM – egg-white cervical mucus. When you are not fertile, the mucus is sticky, cloudy, and doesn’t stretch.
How do I test my cervical mucus?
Watching the changes in the amount and consistency of your cervical mucus can help you understand your cycle. Here’s how it works: check your secretions before and after urinating by wiping with toilet paper. Alternatively, you can insert a clean finger into your vagina to obtain a sample of mucus. Observe (and record) the consistency of the mucus, and use this chart to identify where you are in your cycle. Your mucus can be cloudy, white, yellowish, or clear. It can have either a sticky or stretchy consistency. Use your thumb and forefinger to see if the mucus stretches.
| Cycle timing (approx) | Consistency of mucus | Fertility |
| Day 5 | No noticeable mucus | Not fertile |
| Day 5-8 | No noticeable mucus | Not fertile |
| Day 8-12 | Minimal, cloudy, sticky secretions | Not fertile |
| Day 13-15 | Abundant, clear, wet, stretchy “egg-white” mucus | Fertile window – Before and during ovulation |
| Day 16-28 | No noticeable mucus | Not fertile |
You are most fertile on the days when you have abundant, stretchy mucus. This is not a foolproof method to prevent pregnancy.
References