The menstrual cycle typically begins when a female hits puberty. The age at which it begins, or when one hits puberty, is not the same for everyone. Let’s take a look at the normal menstrual cycle. For better understanding, we recommend checking out our previous article on the female reproductive system first, if you haven’t already.
What is the menstrual cycle?
The reproductive years of a female are normally characterized by monthly rhythmic changes in hormone levels, and corresponding changes in the reproductive or sex organs. This rhythmic pattern is called the menstrual cycle. The most striking feature of the menstrual cycle is menstruation, even though these words are often incorrectly used synonymously.
The word menstrual comes from the Latin word mensis, which means monthly. This is because the duration of the menstrual cycle is 28 days on average, although it varies from person to person. A length of 21 to 35 days is considered normal.
Menstruation usually begins between the ages of 11 to 16. If it occurs before 11, or does not occur after 16, there could be an underlying problem.
Periods, or menstruation, start on the first day of the menstrual cycle and usually last for 2 to 7 days. In the days following the periods, different phases occur to prepare the body for a potential pregnancy, and constant changes take place throughout this time. Let’s get into the different phases of the menstrual cycle and the changes that occur in each phase.
Follicular phase
At birth, the ovaries contain around 1 to 2 million immature eggs (known as primary oocytes). These do not develop until puberty. When puberty begins, only 300,000 to 400,000 oocytes remain in the ovaries. Of these, only 400 to 500 turn into primordial follicles that develop enough to expel in the form of an ovum, which can be fertilized and lead to a pregnancy. The decline in this ovarian reserve of eggs marks the end of reproductive capability or the beginning of menopause.
The follicular phase starts after menstruation. During this phase, follicles grow and mature in the ovaries, and ultimately get ready to release an egg, known as the ovum. This is caused by two hormones - follicle-stimulating hormone (FSH) and luteinizing hormone (LH) - released by the pituitary gland in the brain, which is in turn influenced by another hormone in the brain called gonadotropin-releasing hormone (GnRH).
FSH and LH stimulate the growth of multiple eggs that turn into fluid-filled follicles. The two hormones also increase the production of estrogen by these follicles, which is responsible for growth and thickening of the endometrium in the uterus. The rising levels of estrogen act as a feedback mechanism to the brain that senses the increased estrogen levels in the blood and stops the secretion of FSH to make sure only the needed number of follicles are generated. As the follicular phase progresses, only one follicle becomes dominant, while others ultimately degenerate. The dominant follicle is called the Graafian follicle and is responsible for releasing the egg during ovulatory phase.
Ovulatory phase
During the end of the follicular phase, the high estrogen levels trigger a tremendous release of LH, known as LH surge. This causes the Graafian follicle to “burst” open, thereby releasing the egg. This process in which a mature egg is released from the ovary is called ovulation. The released egg or ovum is then collected from the ovary and transported into the fallopian tubes, where it stays for 12 to 24 hours and can be fertilized by sperm, allowing for pregnancy to occur. Although it is hard to predict when ovulation might happen, it typically takes place 14 days after the first day of the period or menstrual cycle. Some women may experience symptoms such as fever, pain, headaches, breast soreness, etc., during this phase. The cervical mucus and discharge become more slippery and clear, making it easier for any sperm to travel to the egg for fertilization.
Luteal phase
Right after the release of the egg or ovulation, the female body anticipates a possible pregnancy, and starts preparing for it. The remnant of the ruptured Graafian follicle in the ovary turns into corpus luteum, which secretes progesterone. The progesterone is responsible for maintaining the endometrium (the tissue lining the uterus), and facilitating the implantation of a zygote (the egg fertilized by a sperm) in the uterus. The increased estrogen and progesterone work together to prepare the uterus for implantation should fertilization occur.
As soon as implantation is completed, the zygote releases a hormone called human chorionic gonadotropin (hcG), which is used by pregnancy tests to determine pregnancy.
If fertilization does not take place, the corpus luteum degenerates and the resulting decline in progesterone causes the endometrium to shed, marking the onset of menstruation or the period.
The luteal phase is fixed in duration and always lasts for 14 days after which menstruation commences. Some women experience premenstrual syndrome (PMS) near the end of this phase. Headaches, mood changes, acne, and other symptoms are common during PMS.
Menstrual phase
The end of the luteal phase is followed by the shedding of thickened endometrial tissue. Menstruation, commonly known as periods or menses, is the shedding of the endometrium in the form of bleeding. This phase lasts for 2 to 7 days and is also marked by mild to severe pelvic pain, discomfort, and cramps. Although some level of discomfort is considered normal during this time, if one experiences prolonged periods lasting more than 7 days, heavy and painful periods, or any other severe symptoms, then one might be suffering from abnormal menstruation (periods) or abnormal uterine bleeding. The menstrual cycle is unique for each individual, and not everyone experiences it the same way. Many women experience changes in the menstrual cycle, which can sometimes be a sign of an underlying health problem. If you suspect having abnormal periods or cycle, it is advisable to consult a doctor to seek proper diagnosis and treatment.
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References
Richard L. Drake, A. Wayne Vogl, Adam W. M. Mitchell, Gray 39's Anatomy (4th edition.). Elsevier Inc., 2020. (pg. 415-521)
John Edward Hall, Michael Edward Hall, Guyton and Hall textbook of medical physiology (14th edition), Elsevier Inc., 2021. (pg. 1027-1044)
Kasper, D. L., et al. Harrison's principles of internal medicine (20th edition.). New York: McGraw Hill Education., 2018. (pg. 976-978)
Ralston, Stuart H. Davidson’s Principles and Practice of Medicine. 23rd ed. Elsevier Health Sciences, 2018. (pg. 1290-1293)