The Menstrual cycle
On the 28th of May we celebrate Menstrual Hygiene Day in order to raise awareness about menstruation and the importance of good menstrual hygiene management. In honor of this special day, we dedicate the whole month of May to this important topic, one that occurs naturally in half of the global population.
This week, we will be talking about the menstrual cycle, a complex and recurrent process that allows women to conceive (become pregnant). Which hormones control the menstrual cycle and why is it called a cycle? Let’s find out!
The menstrual cycle represents a series of natural changes in female hormone production that make ovulation (when an egg is released from the ovary) and menstruation (monthly vaginal bleeding due to the uterus shedding part of its inner lining when the egg is not fertilized with sperm) possible. This cycle is possible due to the female reproductive system (a complex multi-organ system that involves the ovaries, the fallopian tubes, the uterus (uterine inner lining and cervix), vagina, hypothalamus (region of the brain responsible for releasing hormones and regulating the body temperature) and pituitary gland (the gland that is responsible for regulating vital body functions and general wellbeing).
Women usually have a long reproductive period of approximately 36 years, as the menstrual cycle begins at puberty (can vary from 10 to 16 years of age) and ends at menopause (average age of 51). The length of one menstrual cycle can differ but is around 28 days on average, counted from the start of the menstrual period until the start of the next one.
The menstrual cycle is a complex process, controlled and regulated by hormones, mainly by estrogen, progesterone, the stimulating follicle hormone (FSH), luteinizing hormone (LH), and is comprised of 2 phases, the follicular and the luteal phase.
Phase 1. The Follicular phase
It represents the first phase of the menstrual cycle, occurring from day 0 - 14 of the menstrual cycle (considering an average duration of 28 days for the whole menstruation cycle)
During this phase, the levels of estrogen increase, which stimulates the development of the ovarian follicle (a sac in the ovaries that contains an egg for fertilization) and the growth of endometrial lining inside the uterus
In this phase, a suitable environment for the possible incoming sperm is being created, as channels within the cervix begin to appear which will facilitate the entry of sperm cells
On the last day of this phase, day 14, ovulation starts; considering a 28 cycle, ovulation happens 14 days before the menstruation
During ovulation, high levels of estrogen stimulate the production of the follicle hormone (FSH) and luteinizing hormone (LH). As a result, the mature follicle breaks and releases the egg.
Ovulation is a normal process that is represented by the rupture and release of an egg from the ovary into the fallopian tube where it has the potential to become fertilized and develop into a pregnancy
Phase 2. The Luteal phase
It represent the second phase of the menstrual cycle, occurring from day 14 - 28
In this period, the luteinizing hormone (LH) prepares the corpus luteum (see explanation below) and the inner lining of the uterus for a possible fertilization of the egg
The corpus luteum is a structure formed in the ovary at the site of the ruptured mature follicle; its function is to produce estrogen and progesterone that will provide a suitable environment for the egg to be fertilized with a sperm cell and to develop into a fetus
Progesterone is a hormone that is responsible for preparing the uterine inner lining for the uterine implantation of the fertilized egg and maintenance of pregnancy
If an egg gets fertilized and implanted into the uterine lining, the corpus luteum secretes progesterone in early pregnancy until the placenta (organ that develops during pregnancy inside the uterus, with the role of providing oxygen and nutrients to the growing baby) develops and takes over progesterone production for the remainder of the pregnancy
If the egg doesn’t get fertilized, the corpus luteum will stop releasing estrogen and progesterone
Once the hormone levels start decreasing, the inner lining of the uterus, which thickened in the follicular phase due to the high levels of hormones in order to support a pregnancy, returns to its previous form by shedding (discarding) its extra tissue
This process of shedding extra tissue of the uterine inner lining is called menses, or menstruation
Happens when the egg that was released during ovulation does not get fertilized by sperm and consequently, pregnancy does not occur
During the menstrual cycle, the uterus gets ready for a possible pregnancy by thickening its inner lining. When the pregnancy does not occur, the uterus starts shedding its extra tissue of the uterine inner lining through the vagina. It then comes out as a combination of blood, tissue and blood clots, aka the menstrual bleeding is the external symptom of the menstrual cycle.
The first day of menstruation is considered the first phase of the follicular phase, mentioned above
The usual duration of menstrual bleeding is 3-5 days, but it can range from 1 - 8 days, with the heaviest flow on day 2
The average amount of blood lost during menstruation is 30 mL, but it can range from slight spotting to 80 mL. Loss of more than 80 mL of blood is not considered normal and a doctor should be consulted
Many factors can affect the amount of blood released, such as medication, blood disorders, disorders of blood clotting, the thickness of the uterine inner lining
Menstruation is age-dependent. With increasing age, the period of menstruating days can shorten and the volume of blood can increase
Men can detect the fertile period of women (oestrus) using physiological (fertile scent) or behavioral cues
The fertile period is 5 days before ovulation and the day of ovulation, but the most fertile day is the one before ovulation
Long menstrual cycles (30 or 31 days) and periods of 5 days are linked to the highest probability of pregnancy
The menstrual cycle and the decline of the uterine inner lining is observed in many mammals, but only humans and few other species lose blood
It stops during pregnancy and even after giving birth if the mother is breastfeeding
Home tests that measure ovulation work by measuring the luteinizing hormone (LH) levels in urine, which are high exactly before ovulation
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