The phases and hormones of the menstrual cycle.

Illustration by Emma Günther

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What is the menstrual cycle?

More than just your period

Top things to know about the menstrual cycle

  • The menstrual cycle starts on the first day of the period and ends the day before the following period begins 

  • The uterus and ovaries both experience changes throughout the cycle

  • Ovulation happens when an ovary releases an egg

  • Hormones send signals back and forth between the brain, the ovaries, and uterus to make the cycles and their phases happen

There are many reasons why you may be interested in learning more about your menstrual cycle. Maybe you have recently started your period or are in the process of choosing a birth control method. You could be experiencing changes in your cycle and you would like to better understand why. 

No matter the reason, this cycle (or repeat of events) is a part of you and your everyday life. There is increased freedom when you better understand your menstrual cycle. You have a greater control of preventing pregnancies, conceiving, and in your overall experience of health and your daily activities.

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The menstrual cycle is not just a period

The menstrual cycle is more than just the period. It is a chain of activities in the brain, ovaries, and uterus linked to hormones; the chemical signals sent through the blood from one part of the body to another (1). The cycle is divided into two smaller cycles: the uterine cycle and the ovarian cycle (1). 

The menstrual cycle is broken into phases

Both the uterine cycle and ovarian cycle are divided into different phases, in other words different stages (1). Different events occur during each phase. There are three phases in the uterine cycle: menstruation, proliferative phase, and secretory (before period bleeding) (1). There are also three phases in the ovarian cycle: follicular (before ovulation), ovulation (when an egg is released from an ovary), and luteal (after ovulation) (1).

Your body is not a clock; there is variation in the menstrual cycle

The menstrual cycle is not always perfectly predictable (1). The length of menstrual cycles may vary from cycle to cycle (1,2). Even in people with fairly predictable periods, ovulation may not happen on the exact same day every cycle (3). This can make it hard to predict (1). 

Sometimes ovulation may not happen at all (anovulation) (1). Anovulation is more common during the first years of having a period and when you cycle is coming to an end during (peri)menopause (1). It can also happen while breastfeeding or with certain conditions, such as polycystic ovarian syndrome (PCOS) (1,4). Sometimes ovulation may not happen but a regular period may still happen and there is not a known cause (4).

The menstrual cycle is a very sophisticated process; it can often feel very complex. To help simplify this topic, we will talk about what happens in a menstrual cycle in a linear fashion assuming that ovulation is happening, but every body is unique. There isn’t really a “typical” menstrual cycle. 

Menstrual cycles vary 

A menstrual cycle starts on the first day of the period and ends at the start of the next period. An entire menstrual cycle usually lasts between 24 and 38 days (2), but menstrual cycles can vary from person to person, cycle to cycle, and may also change over the years. 

Cycle lengths change between menarche (when periods first start during puberty) and menopause (when periods stop permanently) (5,6).

Some people notice changes in their hair, skin, poop, chronic disease symptoms, mood, migraine headaches, or even in the way they experience sex at different points in the menstrual cycle.

Understanding the menstrual cycle is important because it can impact the body from head to toe. People use different methods of cycle tracking to understand their experiences, conceive, and prevent pregnancy. Hormonal methods of birth control impact the cycle by either preventing ovulation, changing the lining of the uterus, or both. 

Before Ovulation

1. Uterine Cycle: 

This cycle physically changes the uterus. 

Menstruation (the period)

When: During the period – from the time vaginal bleeding starts to the time it ends.

What: Blood from the previous cycle  from inside the uterus is shed through the cervix and vagina. The uterine lining, called the endometrium, is thinnest during this phase. A typical period may last up to 8 days (2), but on average lasts about five or six (7).

Hormones: Levels of estrogen and progesterone are typically at their lowest. This causes the top layers of the lining to release and leave the body.

Proliferative phase

When: From the end of the period until ovulation.

What: Proliferative means growing quickly. The uterus builds up a thick inner lining while the ovaries prepare eggs for release (oocytes) (8). The uterus thickens so a potential fertilized egg can implant and grow. 

Hormones: Estrogen typically rises during this phase. This signals the uterine lining to grow.

2. Ovarian Cycle:

This cycle physically changes the ovaries. 

Follicular phase

When: From the start of the period until ovulation.

What: Throughout the menstrual cycle, there are multiple follicles (fluid filled sacs containing eggs) at different stages of development in both ovaries (10,11). The period ends about halfway through this phase. One follicle in one of the ovaries becomes the largest of all the follicles. It is called the dominant follicle and is about the size of a grain of sand ( 1-2 cm or 0.4-.8 in) (11,12). This follicle is the one prepared to be released at ovulation. For most people, the follicular phase lasts 10-22 days, but this can vary from cycle-to-cycle (7).

Hormones: The pituitary gland (a small area at the base of the brain that makes hormones) produces a hormone called follicle stimulating hormone (FSH). FSH tells the ovaries to prepare an egg for ovulation (release of an egg from the ovary). The dominant follicle produces estrogen as it grows (13), which peaks just before ovulation happens (12). 

Ovulation

When: About midway through the menstrual cycle, or 13 to 15 days before the start of the next period (14). This can change cycle-to-cycle. Ovulation divides the follicular and the luteal phases of the ovarian cycle as well as the proliferative and secretory phases of the uterine cycle). 

What: The dominant follicle reaches about 2 cm (0.8 in). When it bursts an egg leaves the ovary and enters the fallopian tube (10,12). The release of an egg from an ovary is called ovulation.

Hormones: The dominant follicle in the ovary produces more and more estrogen as it grows larger. When estrogen levels are high enough, they send a signal to the brain. The brain then causes a dramatic increase in luteinizing hormone (LH) (9). This spike is what causes the release of the egg to occur (1). Estrogen levels drop right after ovulation (1).

After Ovulation

1. Uterine Cycle:

Secretory Phase

When: From ovulation until the start of the next period.

What: The endometrium (uterine lining)gets ready to support a pregnancy or to break down for menstruation. The cells of the lining make and release many types of chemicals. 

Hormones: Progesterone rises. This causes the uterine lining to stop thickening and prepare for a fertilized egg. Prostaglandins, “PGF2α” and “PGE2”, cause the uterine muscle to cramp. These hormones rise after ovulation and are highest during a period (15,16). Cramping helps start the period. Blood vessels shrink and the uterine lining breaks down (9). The uterine cells produce less of these chemicals if pregnancy happens (17). 

2. Ovarian Cycle:

Luteal Phase

When: From ovulation until the start of the next period. The luteal phase lasts about 14 days, but between 9 and 16 days is common (7,14).

What: After ovulation, the follicle that held the egg turns into something called a corpus luteum (9,18). It makes the hormones progesterone and estrogen to support pregnancy. (9,18). If pregnancy does not happen, the corpus luteum breaks down between 9 and 11 days after ovulation (9).  

Hormones: Progesterone supports early pregnancy if an egg and sperm connect (19). If no pregnancy happens, progesterone will peak and then drop (20). These hormonal changes can contribute to common premenstrual symptoms. Common symptoms are mood changes, headaches, acne, bloating, and breast tenderness. A drop in progesterone and estrogen causes menstruation. 

Menstruation will begin again, and the cycle starts over .

Understanding your menstrual cycle and rhythm increases overall understanding of your body. This can guide you in the usage of hormonal and non-hormonal birth control. Download Clue to track your period.

This article was originally published on December 12, 2018.

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