The menstrual cycle is important for female reproduction, as it is how the body prepares for pregnancy each month. The menstrual cycle normally has a cycle duration of 28 days, and consists of 4 key phases: It starts with the menstruation, also called menses phase, where the uterine lining (endometrium) is shed through the vagina, causing vaginal bleeding. This normally lasts from day one to five. Next, there is the follicular phase, from day 6 to 14. Rising estrogen levels cause the endometrium to grow, preparing for an egg to nest. At the same time, the follicle-stimulating hormone (FSH) causes several ovarian follicles to grow. During day 10 – 14, one of these follicles will become the dominant follicle, which will be released during the third phase, ovulation. The egg travels from the ovary through the fallopian tubes to the uterus, and is ready for fertilization. In the next phase, the luteal phase which lasts from day 15 to 28, the body awaits fertilization. If it happens, a pregnancy starts. In case there is no fertilization, progesterone levels fall, and the uterine lining is shed again, starting at day 1 of the next menstrual cycle.
The entire process of the menstrual cycle is controlled by hormones such as estrogen and progesterone. In case of menstrual disorders, there’s often also changes in the hormonal patterns that are visible, especially if the irregularities have an endocrine (hormonal) cause such as in PCOS.
- Cleveland Clinic. Normal Menstruation. https://my.clevelandclinic.org/health/articles/10132-normal-menstruation (accessed 8. August 2020)
- Johns Hopkins Medicine. Menstrual Cycle: An Overview. https://www.hopkinsmedicine.org/health/wellness-and-prevention/menstrual-cycle-an-overview (accessed 8. August 2020)
- NHS UK. Periods and fertility in the menstrual cycle. https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/