The Four Phases of the Menstrual Cycle.

By Dr. Greta

So this week, we are going to go over the four phases of the menstrual cycle. I am amazed that so many women are never given the opportunity to learn about their menstrual cycle. It is something that is so integral to who we are as women, yet it’s often associated with embarrassment or taboo (think “just tell the boss you’re having women’s troubles and he won’t ask questions”). It’s not often talked about objectively in the same way we talk about other normal and necessary functions like breathing or digestion. 

The menstrual cycle is something that ALL women should know about. Whether you’re trying to conceive or trying not to conceive, if you’re going through menopause, if you’ve just started getting periods or whether or not you’re on contraception — you can benefit immensely from understanding your menstrual cycle. The menstrual cycle is very individual to each woman, but the phases that occur are the same for all women. 

This week we will cover those general phases so we can get to grips with the basics. However, there are so many things in a woman’s life that can alter her cycle and disrupt these phases. This is something we will cover once we understand what the normal phases of the menstrual cycle are. 

Menstrual cycle basics

The average length of a menstrual cycle is 28 days, but as mentioned this can vary from woman to woman. In this post, we will break down what happens over the course of a ‘typical’ 28-day cycle.

The menstrual cycle is under the control of hormones. Hormones are messengers that travel through the body and bring about change in organs. For example, hormone messages are sent between the brain and the ovaries to trigger changes in the menstrual cycle. 

Below I describe the changes that occur in the ovary and uterus during the four phases of the menstrual cycle. If you are unsure of where everything is inside your pelvis, you can brush up on your pelvic anatomy here.

The four phases of the menstrual cycle as described below are the Start, Early, Middle, and Late phases. The illustration below shows a quick overview of how they relate to each other.

Four phases of the menstrual cycle
Here is a brief overview of the early and late phases of the menstrual cycle.

START:

DAYS: From Day 1 (up to day 8)
PHASE: Menstruation
HORMONES: Progesterone levels decline and trigger menstruation 
SIGNS: Period bleeding 

The first day of your cycle is always counted from the first day of menstruation (the day your bleeding starts). This marks the first of the four phases of the menstrual cycle. Menstruation generally lasts between 2–8 days and involves the shedding of the lining of the uterus. This lining, called the endometrium, is the blood and tissue you see during your period and is shed through the vagina.

EARLY:

DAYS: Days 1–14
IN THE OVARY: Follicular phase
IN THE UTERUS: Proliferative phase
HORMONES: Rising estrogen levels (produced by the dominant follicle)
SIGNS: Starts with your period 

Phases of the menstrual cycle
This diagram shows how Menstruation is the start of the early phase and how they overlap.

Early in the cycle, the ovary is in the follicular phase, which starts on Day 1 of a cycle (with menstruation) and lasts up until ovulation. During the follicular phase, one ovary is busy getting an egg ready to be released. There are multiple follicles in the ovary which each contain an egg. Over the course of the follicular phase, one follicle is recruited to be the ‘dominant’ follicle (it’s the chosen one!). That dominant follicle will continue to grow bigger and bigger (up to 2–3cms) over the follicular phase. As it grows bigger, it produces more and more estrogen. 

follicular phase in the ovary
This illustration shows the changes in the ovary during the Early Phase

During this time the uterus is in the proliferative phase and the endometrium is beginning to thicken again. The whole reason the endometrium is thickening is to create an inviting environment for a fertilised egg to implant itself.

proliferative phase of the uterus
This illustration shows the changes in the uterus during the Early Phase

MIDDLE:

DAYS: ~14 
PHASE: Ovulation
HORMONES: Peak estrogen levels cause a luteinising hormone (LH) surge
SIGNS: Stretchy cervical mucus which resembles egg white, and a change in basal body temperature 

Ovulation happens roughly in the middle of a women’s cycle and occurs when the egg is released from the dominant follicle in the ovary. This egg is released into the fallopian tube. Ovulation is important when a woman is trying to get pregnant, as this ovulated egg can become a pregnancy should it meet a sperm in the fallopian tube after it has been released. 

ovulation from the ovary
Here you can see the dominant follicle release the egg from the ovary.

As you recall, in the earlier phase of the cycle the dominant follicle is growing and releasing more and more estrogen. Once the estrogen level peaks, it signals the brain to release a surge of another hormone called luteinising hormone (LH). This LH surge prompts the dominant follicle to release the egg, and, voilà! Ovulation has occurred! 

LATE:

DAYS: 15–28
IN THE OVARY: Luteal phase
IN THE UTERUS: Secretory phase
HORMONES: Rising progesterone levels (produced by the dominant follicle) which either continue to rise or drop
SIGNS: Pre-menstrual symptoms such as tender breasts, headaches, and acne

During the latter half of your cycle, the ovary is in the luteal phase, which starts after ovulation has occurred and lasts up until the start of the next period. During the luteal phase, the empty dominant follicle (which has already released its egg during ovulation) transforms into a Corpus Luteum.

luteal phase of the ovary
This illustration shows the changes in the ovary during the Late Phase.

The Corpus Luteum releases progesterone to help bring about changes in the uterus depending on whether a pregnancy will occur. If the egg has met with a sperm in the fallopian tube, the Corpus Luteum will continue to release progesterone to help support that pregnancy. If the egg is not met by sperm and no pregnancy occurs, the Corpus Luteum will start to break down and the progesterone levels will fall. This fall in progesterone will eventually trigger menstruation to occur again, taking us back to the start of the menstrual cycle. 

The rising progesterone levels after ovulation can cause some women to experience pre-menstrual symptoms such as tender breasts, acne, and headaches. 

During this late phase of the cycle, the uterus is in the secretory phase and the newly thickened lining of the uterus (endometrium) is doing one of two things depending on whether a woman has conceived. It will either be preparing to support and bear a pregnancy or if pregnancy has not occurred (ie; the ovulated egg has not been met with a sperm) then it is getting ready to shed its lining again, which takes us full circle to the start of the cycle when menstruation occurs.  

Secretory phase of the uterus
This illustration shows the changes in the uterus during the Late Phase

Menstrual cycles differ from woman to woman.

While the above is the basis of the menstrual cycle, the phase lengths are only rough estimates as most women will have a slightly variable cycle which can be as short as 21 days or as long as 35 (or perhaps even longer if there are other hormonal issues). This can even differ from cycle to cycle. But the general principles still apply to most women. Menstruation marks the start of the cycle and ovulation marks the middle of the cycle. On either side of ovulation, the ovary and uterus are either in an early phase or late phase of the cycle.

Let’s summarise the four phases of the menstrual cycle below. The first illustration shows where each phase lies in the cycle and how each phase relates to the other.

four phases of the menstrual cycle

The next illustration summarises the changes that occur in the Ovary during the early and late phases of the cycle.

ovarian cycle

And finally, this last illustration summarises the changes that occur in the Uterus to the Endometrium during the phases of the cycle.

uterine cycle

I hope this gives you a little bit of a deeper understanding of what happens during your menstrual cycle. It is certainly important to understand when certain things are occurring in your body. You can correlate this with how you are feeling and what symptoms and signs you are experiencing. It is also useful to know if you are trying to get pregnant or avoid pregnancy.

Once you have your head around the basic flow of your cycle, we will start to look into the cycle at a deeper level and we will also explore things that can impact your cycle in both a positive and negative way. Until then, why don’t you start tuning in with your own cycle and try to match up what you’ve learned about what’s happening in your body with what you are experiencing.

Written and illustrated by Dr. Greta

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