Before diving into the menstrual cycle, it is important to recognize the different parts of women’s reproductive organs [2]:
The entrance is called vulva, and it opens to the vagina, where penetration occurs.
The cervix separates the vagina and the uterus.
The uterus (commonly known as the womb) is where the fetus develops
The endometrium is the lining inside the uterus where the fertilized egg implants at the beginning of a pregnancy.
The fallopian tubes is where the eggs can get fertilized by sperm. It connects the uterus and the ovaries.
There are two ovaries where the eggs are stored and developed.
On average, a menstrual cycle occurs every 28 days. However, between 21 and 38 days is also considered normal [1]. The menstrual cycle actually has two different events happening at the same time: the ovarian cycle and the uterine cycle. This whole process prepares the body for a potential pregnancy each month.
Ovarian cycle
The ovarian cycle happens in the ovaries, and it’s divided into three different phases where the egg matures and is ejected into the fallopian tubes to be fertilized.
1. Follicular phase
A follicle is a sac that contains one egg and its necessary fluid [3]. During the first 13 days of the cycle, follicle-stimulating hormone (FSH) matures the follicle. As it matures, it secretes estrogen.
2. Ovulatory phase
When there is enough estrogen (usually at day 14), more FSH and luteinizing hormone (LH) are produced. Ovulation happens at the peak of these two hormones [4-5]. In this phase, the mature follicle ruptures and releases the egg into the fallopian tubes to be fertilized.
3. Luteal phase
From day 15 to the end of the cycle, both FSH and LH levels are low to prevent the maturation of more follicles [4]. Most importantly, these low levels cause the remaining parts of the follicle (corpus luteum) to produce progesterone – the hormone in charge of making the endometrium ready for implantation [5].
Uterine cycle
This cycle begins with menstruation, which goes hand in hand with the follicular phase. During the rest of the cycle, a new endometrium grows to create an optimal environment for the implantation of the fertilized egg and the start of pregnancy.
1. Menstruation
This event happens when the implantation into the endometrium is not successful. The corpus luteum starts degenerating, which causes a drop in estrogen and progesterone levels [6], and the endometrial lining to shed, leading to bleeding for around 5 days.
2. Proliferative phase
Due to the maturation of the follicle during the follicular phase, estrogen levels gradually rise, and the endometrium starts to thicken [6]. This usually happens between day 5 to 14 in the cycle, until the ovulatory phase.
3. Secretory phase
From day 14 to 28, high estrogen and progesterone levels stimulate the growth of the endometrium, preparing it for a potential pregnancy [6].
When pregnancy doesn’t happen, menstruation occurs, beginning both the ovarian and uterine cycles again. Interestingly, premenstrual symptoms such as irritability, fatigue, tender breast, bloating, cramping, and more, may be due to estrogen and progesterone levels falling so drastically [7] rather than an increase in hormones.
On the contrary, if pregnancy does occur, the fertilized egg would implant in the endometrium and the placenta will begin to develop. In this case, the corpus luteum will not degenerate, and the levels of estrogen and progesterone will stay elevated. Moreover, the placenta will start secreting the human chorionic gonadotropin (hCG) hormone very early on. The hCG hormone triggers the body to create even more progesterone and estrogen, stopping the menstrual cycle from starting and thickening the uterine lining to support the placenta [8].
There are many things happening in females bodies every month. Upcoming posts will discuss the development of pregnancy tests and contraceptives, as well as how stress can affect the menstrual cycle.
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Written by: Maria
Edited by: Natasha
Figures created by: Adrian
BioDecoded is a volunteer group committed to sharing accurate scientific information. We cannot offer any specific health advice. If you have any doubts about your personal health, please speak with your healthcare professional or family physician. Your doctor can revise your medical history and advise you on the best path to follow. If you have any questions about this topic or would like to learn more, please comment below or send us your questions.
References:
Menstrual Cycle (Normal Menstruation): Overview & Phases (2023). Cleveland Clinic. Available at: https://my.clevelandclinic.org/health/articles/10132-menstrual-cycle (Accessed: 28 January 2023).
Periods and fertility in the menstrual cycle (2018). National Health Service UK. Available at: https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/ (Accessed: 28 January 2023).
Dictionary (2023). National Cancer Institute. Available at: https://www.cancer.gov/publications/dictionaries/cancer-terms/def/ovarian-follicle (Accessed: 28 January 2023).
Orlowski, M. and Sarao, M. (2022) "Physiology, Follicle Stimulating Hormone", StatPearls Publishing. Available at: https://www.ncbi.nlm.nih.gov/books/NBK535442/
Ovarian Cycle (2023). Medicine Libre Texts. Available at: https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/26%3A_The_Reproductive_System/26.6%3A_Physiology_of_the_Female_Reproductive_System/26.6B%3A_Ovarian_Cycle (Accessed: 28 January 2023).
Uterine (mentrual) cycle (2023). Medicine Libre Texts. Available at: https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Anatomy_and_Physiology_(Boundless)/26%3A_The_Reproductive_System/26.6%3A_Physiology_of_the_Female_Reproductive_System/26.6C%3A_Uterine_(Menstrual)_Cycle (Accessed: 28 January 2023).
Premenstrual syndrome (PMS) (2023). Office on Women's Health. Available at: https://www.womenshealth.gov/menstrual-cycle/premenstrual-syndrome (Accessed: 28 January 2023).
Human Chorionic Gonadotropin: Hormone, Purpose & Levels (2023). Cleveland Clinic. Available at: https://my.clevelandclinic.org/health/articles/22489-human-chorionic-gonadotropin (Accessed: 28 January 2023).
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