The menstrual cycle is the monthly series of changes the female body goes through to prepare a woman for possible pregnancy. They are usually present for a large part of a woman’s life, starting at puberty, and ending at menopause.
It is a complicated process to regulate, with many parts of the body playing a role, such as the uterus, the ovaries, and the brain. The cycle begins with the first day of menstrual bleeding (menstruation) and is divided into phases, with a normal cycle length being between 24 and 38 days. The exact length of someone’s cycle can vary from one month to the next.
During one cycle, an egg matures in the ovaries, and is released. The lining of the uterus, known as the endometrium, grows and swells up to allow a fertilized egg to implant. If the egg has been fertilized by a man’s sperm, it attaches to the endometrium in the uterus, and starts to grow into a fetus. If the egg doesn’t become fertilized, it won’t attach, which leads to the lining of the uterus shedding and being released through the vagina as menstrual blood.
Menstrual cycle phases
When describing the menstrual cycle, we look at the changes that occur in the ovary, and use these to divide it into three phases:
Follicular Phase:
This phase starts on the first day of menstruation, and is when the follicles, small fluid-filled sacs found inside a woman’s ovaries, grow and develop. Normally, during each cycle, only one of these follicles develops fully and it contains the future egg that will be released during ovulation. The follicular phase usually lasts around 13 days, with menstruation lasting between 4 and 8 days at the start of the follicular phase.
Ovulatory Phase:
This is the shortest phase of the cycle, lasting around 2 days. During this phase, the follicle bursts, releasing the egg into the duct that connects the ovary to the uterus.
Luteal Phase:
The ruptured follicle that is left behind in the ovary becomes a structure called the corpus luteum, which produces progesterone. This phase usually lasts around 14 days, during which, two things can happen: If the egg doesn’t become fertilized or doesn’t attach to the endometrium, the corpus luteum starts to degenerate, and progesterone levels start to decrease halfway through this phase. However, if the fertilized egg implants correctly to the endometrium, the corpus luteum keeps working to produce progesterone, which helps promote the pregnancy.
Hormonal changes during the menstrual cycle
Today, hormones are given a bad rap. They’re blamed for many things, and people try to avoid taking them unless it is absolutely necessary. But do we really know what a hormone is?
Hormones are chemical messengers that control and coordinate our body’s activities. They travel through the bloodstream to the tissues and organs, and there are different types of hormones that interact with each other to regulate all essential body functions, not just our reproductive system.
Exactly how they work and interact with each other is a complex process with many layers, but we are focusing on their impact in the female reproductive system.
The first thing to understand is that the female reproductive system is regulated primarily by hormones that are produced in the brain, and the ovaries. In one part of the brain, the hypothalamus, a hormone called GnRH is produced and acts as a messenger to tell the pituitary gland (also a part of the brain) to produce the hormones LH and FSH. LH and FSH travel in the blood to the ovaries, stimulating it to produce the female sex hormones estrogens and progesterone.
These four are considered the menstrual cycle hormones (LH, FSH, estrogen and progesterone), and lead to the different phases of the cycle. But it does not work as an on and off switch. Each hormone has a particular role, and its effect can vary depending on the level or amount of each hormone.
Hormone cycle
Through the cycle, the hormones interact with each other, leading the changes in the endometrium or uterine lining:
At the beginning of the follicular phase, estrogen and progesterone levels are low, which causes the endometrium to shed. During this phase, FSH is in charge of stimulating the growth and development of the follicles in the ovary.
After the follicular phase is over, estrogen and progesterone then cause the endometrium to grow and thicken with blood vessels. The endometrium reaches full growth at the end of the ovulatory phase, time during which there is also a big surge in luteinizing hormone, which is what signals for the egg to be released from the follicle. If a fertilized egg attaches correctly to the endometrium, it does not shed, and the fetus begins to develop. On the other hand, if there is no fertilization, the endometrium sheds again, marking the beginning of the next cycle.
After this, during the luteal phase, progesterone levels increase and then decrease towards the end of the phase.
Symptoms during the menstrual cycle
As we have seen, hormone levels change during the menstrual cycle and this has an effect throughout the entire body, not just the reproductive system.
Some of the symptoms that we associate with the menstrual cycle can be caused by changes in hormone levels. Premenstrual Syndrome (PMS) is a group of symptoms that some women experience before the start of, or during their period. These include changes in the woman’s mood, like increased feeling of sadness, or nervousness, mood swings or a short temper. PMS affects around 48% of women, and the symptoms and effects are different for each person. The exact cause of PMS symptoms is unknown, but research suggests that progesterone is probably one of the causes of these changes in mood. However, each person has a different sensitivity to progesterone, which can be affected by factors such as environmental changes or even age. There are also physical symptoms like breast tenderness, cramps, headaches, acne, and bloating.
Hormonal changes during the menstrual cycle can also affect sleep quality, both high level and low levels of progesterone have been associated to sleep problems. On the other hand, higher levels of estrogen have positive effects on sleep.
Estrogen is also known to participate in fat metabolism, promoting the formation of “good cholesterol” (HDL) and helps to keep “bad cholesterol” (LDL) at low levels. Another of its benefits has to do with making the skin have a healthy appearance, by supporting the formation of collagen and blood vessels in it.
Although it might not seem logical, estrogen can be responsible for changes in bone health. It helps prevent calcium loss and contributes to maintaining a healthy bone mass. When a woman goes into menopause, her estrogen levels decrease, and this can play a role in the development of osteoporosis, a disease caused by loss of bone mass and loss of bone mineral density (BMD).