The menstrual cycle begins with the hypothalamus releasing  gonadotropin-releasing hormone (GnRH).  GnRH then causes the anterior pituitary gland to release follicle stimulating hormone (FSH), which stimulates the development of follicles. The anterior pituitary gland also releases  luteinizing hormone (LH), which also stimulates follicles. Both FSH and LH are integral in having an egg develop during the follicular phase. When the ovaries register that the pituitary gland is releasing FSH, it starts to stimulate the follicles to grow. As the follicles develop, the cells surrounding them release estrogen. Several follicles grow within the ovary, but only one typically matures into a Graafian follicle, which is the follicle that will release the mature egg, or ovum. The other follicles die and do not release eggs. As the Graafian follicle grows it releases estrogen. As the estrogen levels increase, there is a slight decrease in the amount of FSH and LH

Once the follicle is ready to be released, estrogen levels increase drastically, and the anterior pituitary gland produces a massive amount of luteinizing hormone, which is known as the LH surge, which occurs approximately during day 13 of the menstrual cycle. The LH surge is integral in releasing the egg from the Graafian follicle and having it travel into the fallopian tubes for potential fertilization. The process by which the mature egg is released is called ovulation, which marks the ends of the follicular phase.

Central Purpose: To release an ovum (or mature egg)

Description: Approximately 8 to 20 hours after the LH surge, the increase in luteinizing hormone contributes to the weakening of the wall of the ovarian follicle, which causes it to be ruptured and the ovum (or mature egg) to be released. This is known as ovulation and it occurs on approximately day 14 of the menstrual cycle or the midpoint of the menstrual cycle. . A symptom of ovulation is a slight increase in basal body temperature (BBT) by roughly to .3 to 1.1 degrees Fahrenheit.  Another symptom of ovulation is a change in the consistency and color of cervical mucus. During ovulation, cervical mucus resembles egg whites in terms of texture, stretchiness, and color. 

Central Purpose:  To thicken the lining of the uterus to prepare for the successful implantation and nourishment of a potential fertilized egg

After the Graafian follicle releases the ovum during ovulation, the remains of the Graafian follicle become a benign cyst called the Corpus Luteum. The Corpus Luteum secretes high levels of progesterone, which thickens and maintains the endometrium (or lining of the uterus) to prepare for a potentially fertilized ovum. Progesterone also causes cervical mucus to thicken, which makes it more difficult for sperm to swim through.  The Corpus Luteum also secretes estrogen. Once the ovum is released, finger-like structures called fimbriae that are located at the ends of fallopian tubes near the ovaries sweep the egg into a fallopian tube. The ovum is swept along by cilia (which are hair-like structures within the fallopian tubes) and by contractions in the walls of the fallopian tube.  If the egg is fertilized, then the embryo secretes Human Chorionic Gonadotropin (HCG), which keeps the Corpus Luteum alive. HCG is what’s tested for in a pregnancy test. 

If the egg is not fertilized and HCG is not produced, the Corpus Luteum degenerates and stops producing estrogen and progesterone. This decrease in estrogen and progesterone. There is also a decrease in FSH. The decrease in progesterone and estrogen results in the endometrium being broken down inside the uterus. The shedding endometrium travels through the cervix and eventually exits the body through the vagina. This process is known as menstruation, menstrual flow, menses, or a period. Thus, the luteal phase ends when menstruation begins. Menstruation can last anywhere from 2 to 8 days and marks the beginning of a new menstrual cycle and the start of a new follicular phase.

APA Dictionary of Psychology. (n.d.). Retrieved from https://dictionary.apa.org/hypothalamus ssociation. (n.d.). Apa Dictionary of Psychology. American Psychological Association. Retrieved March 21, 2022, from https://dictionary.apa.org/hypothalamus

Gotter, A. (2018, May 07). Low Progesterone: Complications, Causes, and More. Retrieved from https://www.healthline.com/health/womens-health/low-progesterone#:~:text=Progesterone helps to regulate your,levels drop and menstruation begins.

Hormone Imbalance, Menstrual Cycles & Hormone Testing. (2019, April 12). Retrieved from https://womeninbalance.org/about-hormone-imbalance/,

Knudtson, J., & McLaughlin, J. E. (2022, March 30). Menstrual Cycle - Women's Health Issues. Retrieved from https://www.merckmanuals.com/home/women-s-health-issues/biology-of-the-female-reproductive-system/menstrual-cycle

Menstrual cycle. (n.d.). Retrieved from https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/menstrual-cycle

Menstrual cycle: What's normal, what's not. (2021, April 29). Retrieved from https://www.mayoclinic.org/healthy-lifestyle/womens-health/in-depth/menstrual-cycle/art-20047186

Menstruation – amenorrhoea. (n.d.). Retrieved from https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/menstruation-amenorrhoea

Watch how ovulation happens. (2020, June 27). Retrieved from https://www.mayoclinic.org/healthy-lifestyle/womens-health/multimedia/ovulation/vid-20084729#:~:text=The egg travels through the,celled entity called a zygote.

Mayo Clinic Q and A: Irregular periods can be common at first - Mayo Clinic News Network. (2017, December 19). Retrieved from https://newsnetwork.mayoclinic.org/discussion/mayo-clinic-q-and-a-irregular-periods-can-be-common-at-first/#:~:text=For the first few years,body isn't ovulating monthly 

Palmer, A. (2020, March 28). How the menstrual cycle affects acne. Verywell Health. Retrieved April 17, 2022, from https://www.verywellhealth.com/is-premenstrual-acne-real-15686#:~:text=Premenstrual%20Acne%20Symptoms&text=These%20breakouts%20appear%20mostly%20on,menstruate%2C%20according%20to%20various%20studies  

Mayo Foundation for Medical Education and Research. (2022, February 25). Premenstrual syndrome (PMS). Mayo Clinic. Retrieved April 17, 2022, from https://www.mayoclinic.org/diseases-conditions/premenstrual-syndrome/symptoms-causes/syc-20376780 

Mayo Foundation for Medical Education and Research. (2022, January 21). Premenstrual water retention: How to get relief. Mayo Clinic. Retrieved April 17, 2022, from https://www.mayoclinic.org/healthy-lifestyle/womens-health/in-depth/water-retention/art-20044983 

WebMD. (n.d.). Menstrual cramps (dysmenorrhea): Symptoms, causes, treatment, prevention. WebMD. Retrieved April 17, 2022, from https://www.webmd.com/women/menstrual-cramps#:~:text=Menstrual%20cramps%20happen%20because%20of,causes%20your%20pain%20and%20cramping 

Encyclopædia Britannica, inc. (n.d.). Menstruation: Media. Encyclopædia Britannica. Retrieved April 17, 2022, from https://www.britannica.com/science/menstruation/images-videos

Nall, R. (2019, March 26). Cervix before period: How to check, changes throughout cycle, more. Healthline. Retrieved April 17, 2022, from https://www.healthline.com/health/cervix-before-period 

Nall, R. (2019, March 26). Cervix before period: How to check, changes throughout cycle, more. Healthline. Retrieved April 17, 2022, from https://www.healthline.com/health/cervix-before-period 

Basic Anatomy and Physiology of the Uterus . Center for uterine fibroids. (n.d.). Retrieved April 17, 2022, from https://www.fibroids.net/fibroids.html 

Elizabeth Boskey, P. D. (2020, January 27). Fallopian tubes: Anatomy, function, and treatment. Verywell Health. Retrieved April 17, 2022, from https://www.verywellhealth.com/fallopian-tubes-anatomy-4777161 

Boskey, E. (2020, January 27). Fallopian tubes: Anatomy, function, and treatment. Verywell Health. Retrieved April 17, 2022, from https://www.verywellhealth.com/fallopian-tubes-anatomy-4777161 

Home. You and Your Hormones. (n.d.). Retrieved April 17, 2022, from https://www.yourhormones.info/hormones/follicle-stimulating-hormone/ 

NCI Dictionary of Cancer terms. National Cancer Institute. (n.d.). Retrieved April 17, 2022, from https://www.cancer.gov/publications/dictionaries/cancer-terms/def/gonadotropin-releasing-hormone

The Children's Hospital of Philadelphia. (2014, August 24). Menstrual cycle: An overview. Children's Hospital of Philadelphia. Retrieved April 17, 2022, from https://www.chop.edu/conditions-diseases/menstrual-cycle-overview 

Female reproductive system: Structure & Function. Cleveland Clinic. (n.d.). Retrieved April 17, 2022, from https://my.clevelandclinic.org/health/articles/9118-female-reproductive-system 

Conception: How it works. Center for Reproductive Health. (n.d.). Retrieved April 17, 2022, from https://crh.ucsf.edu/fertility/conception#:~:text=Once%20the%20sperm%20have%20entered,for%20up%20to%205%20days. 

UCSF Health. (2021, September 22). The menstrual cycle. ucsfhealth.org. Retrieved April 17, 2022, from https://www.ucsfhealth.org/education/the-menstrual-cycle 

UCSF Health. (2021, September 22). The menstrual cycle. ucsfhealth.org. Retrieved April 17, 2022, from https://www.ucsfhealth.org/education/the-menstrual-cycle 

UCSF Health. (2021, September 22). The menstrual cycle. ucsfhealth.org. Retrieved April 17, 2022, from https://www.ucsfhealth.org/education/the-menstrual-cycle

Cornforth, T. (2021, July 23). How the luteal phase helps conception to occur. Verywell Health. Retrieved April 17, 2022, from https://www.verywellhealth.com/luteal-phase-of-the-menstrual-cycle-3522712#:~:text=During%20the%20luteal%20phase%2C%20estrogen,nourishment%20of%20a%20fertilized%20egg. 

Menstrual Cycle Phases Female Reproductive System Follicular Luteal Proliferative Secretory. (2016). YouTube. Retrieved April 17, 2022, from https://youtu.be/lVPQ_w7ZUTA.