Understanding the Different Phases of The Menstrual Cycle



What is your menstrual cycle?

The menstrual cycle is the female body’s way of preparing for pregnancy each month. It begins with the first day of your period, also referred to as menstruation. This is considered day 1 of your cycle. The menstrual cycle ends when the next period begins.


Throughout the cycle there are fluctuations in chemicals known as hormones. These hormones rise and fall depending on whether your body is preparing to release an egg, aka ovulation, sustain a pregnancy, or in preparation for your next cycle to begin.


These changing hormones can cause different symptoms throughout your cycle, including physical, mental, and emotional symptoms.


How long is the menstrual cycle?

A “normal” menstrual cycle is somewhere between 21-35 days with day 1 of the cycle being the first day of your period. However, the menstrual cycle can be longer than 35 days which is referred to as oligomenorrhea and can often be seen in conditions like PCOS. The cycle can also be shorter than 21 days which is referred to as menorrhagia and is most commonly seen in perimenopause.


If the average length of your menstrual cycles fall outside of that 21-35 day window or if you have troubling symptoms related to your periods or your hormones, contact your doctor for treatment.


What are the phases of the menstrual cycle?

The menstrual cycle can be broken down into three main phases: the follicular phase, ovulation, and the luteal phase.



Follicular Phase

This phase starts on day 1 of your period and continues until ovulation. The follicular phase can be broken down into two subphases: menstruation and proliferation.


Menstruation

This phase begins on day 1 of your period and continues until you are done bleeding.


Menstruation occurs when the egg from your previous cycle was not fertilized. During this phase you are shedding your uterine lining.


Proliferation

The proliferative phase begins the day after your period stops and continues until you ovulate.


The pituitary gland, found in your brain, releases something called follicle stimulating hormone, FSH. FSH then tells your ovaries to start producing follicles, which are little sacs that encase your eggs as they begin to mature. The follicles stimulate the release of estrogen which in turn tells your uterus to begin rebuilding and thickening the uterine lining, aka the endometrium. Only the healthiest egg matures (occasionally two eggs might mature and can become fraternal twins).


Ovulation

Ovulation occurs somewhere between days 13-16 in a normal cycle. Occasionally menstrual cycles are anovulatory, meaning that ovulation doesn’t occur.


The pituitary gland releases luteinizing hormone, LH. This hormone tells your ovary to release the mature egg into the fallopian tube so that it can be fertilized.


Read more about ovulation and why you want it to occur even if you aren’t trying to get pregnant here.


Luteal Phase

The luteal phase begins after ovulation has occurred and continues until you get your next period.


After ovulation, the ruptured follicle becomes what is known as the corpus luteum, which secretes progesterone. Progesterone is essential for maintaining the endometrium integrity for implantation of a fertilized egg. Remember, when the endometrium is shed it is what is known as your period. Therefore, it’s vital that the endometrium remains intact to support a pregnancy.


If fertilization has not occurred, the corpus luteum regresses and progesterone drops, allowing the next menstrual cycle to begin as the endometrial lining is shed again.


Hormones involved in the Phases of the Menstrual Cycle

There are five main hormones involved in your menstrual cycle.


Estrogen: estrogen encourages ovulation to occur.

Progesterone: progesterone prepares the endometrial lining for pregnancy.

Luteinizing hormone: LH is released from the pituitary gland and triggers the ovary to release a mature egg during ovulation.

Follicle stimulating hormone: FSH is released from the pituitary gland and stimulates the ovaries to mature follicles for ovulation.

Testosterone: testosterone is an androgen that supports the menstrual cycle indirectly through increasing libido near the time of ovulation. It’s also necessary for maintaining estrogen and progesterone balance.


Understanding how your menstrual cycle works is essential for having an understanding of your body. If you are experiencing irregular cycles or are having trouble trying to conceive, reach out to your doctor to see what options you have.


Resources:

Mihm, M. Gangooly, S. Muttukrishna, S. The normal menstrual cycle in women. Animal Reproduction Science. 2011.


Reed, B. Carr, B. The normal menstrual cycle and the control of ovulation. Endotext. 2011.


Hillier, S. Reichert, L. Hall, E. Control of Preovulatory follicular estrogen biosynthesis in the human ovary. The Journal of Clinical Endocrinology and Metabolism. 1981.


Bartelmez, G. The phases of the menstrual cycle and their interpretation in terms of the pregnancy cycle. American Journal of Obstetrics and Gynecology. 1957.


Fehring, R. Schneider, M. Raviele, K. Variability in the Phases of the Menstrual Cycle. Journal of Obstetrics, Gynecology, and Neonatal Nursing. 2006.