By Dr. McKenzie Nisbet, ND

Your Menstrual Cycle

YOUR MENSTRUAL CYCLE

This is your menstrual cycle. It’s a spectacular, yet complex system and I want to help you understand it. Understanding your cycle gives you a foundation for taking control of your reproductive health. I’m going to help break down the key players and the phases of this balancing act.

My goal by the end of this post is to have the image below look a bit less daunting.

THE KEY PLAYERS

YOUR HORMONES: These are the  chemical messages that control changes in your body.

• Estrogen – facilitates the growth of the lining of your uterus in preparation for pregnancy
• Progesterone – helps estrogen prepare your body for pregnancy in the second half of your cycle
• FSH (follicle stimulating hormone) – is responsible for the maturation of the egg so it is ready to be fertilized
• LH (luteinizing hormone) – triggers the releasing the egg from your ovary (ovulation)
• GnRH (gonadotropin releasing hormone) – controls the hormones above and is released from your hypothalamus

YOUR GLANDS, ORGANS & MORE: These create, send and receive the hormone messengers (they use hormones to communicate with one another).

• Hypothalamus – produces GnHR and is located in your brain 
• Pituitary gland – produces LH & FSH and is located in your brain 
• Ovaries – the follicle (egg) is made here
  • At ovulation the egg is released from your ovary, leaving behind the corpus luteum
• Fallopian Tubes – the egg travels through this tube to your uterus after ovulation
• Uterus – the functional endometrial lining of your uterus grows during your cycle and is expelled during your period
  • If your egg is fertilized it’s goal will be to implant in the thick lining created during your cycle
• Follicles – these are present in your ovaries
  • Each cycle numerous follicles will begin to mature in each ovary, but only one will be selected to fully mature as the dominant follicle

YOUR CYCLE

Your cycle begins on the first day of your period: “Day 1”.

A normal cycle can be anywhere from 21-35 days, with the average cycle being 28 days. We will use a typical 28 day cycle as the example below.

I’ve separated the cycle into four sections:

• Your Period: the first part of the follicular phase, Day 1, up to day 8
• The Follicular Phase: Day 1-14
• Ovulation: the transition between the follicular phase and the luteal phase
• The Luteal Phase: Day 15-28

Your Period

DAY 1: Your period starts. 

The first day of bleeding marks the first day of your menstrual cycle. The period usually lasts 4-6 days (but 2-8 is considered normal for some women).

DAY 4-6: Your period ends.

Your period is the natural removal of the functional layer of the endometrial lining of your uterus. This layer gets built up during your cycle in preparation for the implantation of a fertilized egg. If there is no implantation your body is triggered to release this extra lining so the cycle can start all over again. We’ll chat more about the hormones in this stage near the end of the post.

There are two phases during your cycle: the FOLLICULAR phase and the LUTEAL phase.

We’ll start with the follicular phase since it comes first.

Follicular Phase (Day 1-14)

• Goal: mature the egg so it is ready for fertilization by the sperm
• Length: around 14 days – from the start of your menstrual cycle (Day 1), until ovulation (Day 14)
• Key Players: Estrogen, FSH, LH

Lets start with FSH. The blue line in the picture above shows the change in FSH throughout the cycle. You can see FSH start to rise at the end of the menstrual cycle and continue to rise at the start of the ‘next’ cycle.

What causes this increase in FSH?

• Estrogen (grey line) and progesterone (purple line) decline at the tail end of your cycle which triggers FSH to start to increase, and
• Your hypothalamus starts producing GnRH in pulses which tells your pituitary to release more FSH

What does this increase in FSH do?

• FSH is the “follicle stimulating hormone”. It is in-charge of maturing the follicles (eggs) so that one is full matured by the time we reach ovulation around day 14. 

During the follicular phase multiple follicles will start to mature in both your ovaries, but only ONE follicle from ONE ovary will fully develop.

• This follicle is called the ‘dominant follicle’ and will be selected by Day 8.
• It sends out signals telling the other follicles to stop maturing. 

DAY 8: At this point you have a high FSH and a maturing dominant follicle.

Around this time estrogen levels start to rise. 

What causes an increase in estrogen?

• Essentially the maturation of the follicle causes this increase in estrogen. As the follicle gets bigger the amount of estrogen will increase. 

What does the increase in estrogen do?

• Estrogen prepares your uterus for the implantation of the fertilized egg. It does this by building up the functional endometrial layer of your uterus. This is the layer that will be expelled during your period, if no pregnancy occurs.

As estrogen increases and the follicle matures, your FSH starts to drop.

Lets shift our focus to LH as we near ovulation. 

LH (pink line) begins to rise slightly in the mid-follicular phase. Notice the huge increase in LH right before ovulation? This is often referred to as the “LH surge”. 

What is LH for and why does it spike?

• LH is released from your pituitary.
  • At first it is released in less frequent bursts, then as we get further into the follicular phase it is released more and more frequently.
  • This increased frequency causes more LH to be detectable in your body at once, so we get that sharp rise in LH: the LH surge. 
• The surge begins roughly 34-36 hours before ovulation and peaks roughly 10-12 hours before ovulation.
• The LH surge is why we often use LH strips (detects LH in the urine) when we are trying to conceive. The rise in LH predicts ovulation so having sex around this time increases your likelihood of conception.

You’ll also notice a small increase in FSH at the time of the LH surge. This is because the LH surge triggers a quick increase in progesterone which triggers a short lived increase in FSH. 

(Just Before) DAY 14: LH surge begins 34-36hrs before ovulation and peaks 10-12hrs before ovulation. Estrogen reaches its highest level and we get a small peak in FSH.  

So what now? We have a LH surge which represents the time of ovulation, but what is actually happening during ovulation?

Ovulation

The surge of LH triggers changes within the follicle which causes the egg (known as the “secondary oocyte”) to be released from your ovary.

This egg now makes its way from your ovary to your fallopian tube.

Your body now takes one of two paths: fertilization or no fertilization. 

1. Fertilization – a sperm successfully fertilizes the egg and they travel to your uterus. In your uterus they will implant themselves into your thick uterine wall which estrogen has created for them. This fertilization causes the creation of hCG (human chorionic gonadotropin) which is what a pregnancy test detects.
2. No fertilization – the egg stays in the fallopian tube for 24-48 hours. If it does not meet a viable sperm it will disintegrate.

Lets assume that no fertilization took place in this example.

DAY 14: Ovulation (egg is released) and we enter the luteal phase. At this point FSH and LH have fallen, estrogen has dipped slightly, and progesterone is on the rise.

Luteal Phase (Day 15-28)

• Goal: maintain the functional endometrial lining so the fertilized egg can implant 
• Length: around 14 days – from ovulation (Day 14) until the end of your period
• Key Players: Progesterone (mainly)

Progesterone is the main hormone in this phase of your cycle.

This rise in progesterone ensures two things:

• Your uterine lining remains thick and healthy in case the egg is fertilized and continues its way to your uterus.
• A mucous plug forms at your cervix so no more sperm can enter your uterus.

But where does the progesterone increase come from?

• The corpus luteum!

When the egg is released from your ovary it leaves behind the rest of the follicle (all of the supporting cells that helped it mature and grow during the follicular phase). These cells grow, join other cells from your ovary and become yellow in colour – hence the name “corpus luteum” (yellow body). 

The corpus luteum releases progesterone which keeps your uterus ready for implantation. The functioning of this group of cells reaches a peak around 8-9 days after ovulation when implantation is expected.

Progesterone is often tested on Day 18-21 of your cycle to catch this peak in it’s production.

DAY 18-21: Progesterone reaches it’s peak and then starts to decline as the corpus luteum starts to breakdown.

After this point the corpus luteum stops receiving signals to produce progesterone and it starts to decline in size. By the end of the luteal phase it becomes scar tissue in your ovary known as the “corpus albicans” (white body). 

This drop in progesterone means the functional endometrial lining of your uterus is no longer being maintained and your period begins.

• Deceased progesterone levels cause the blood supply to this functional layer to change. These changes prevent oxygen and nutrients from reaching this layer. Now that the layer can no longer survive it begins to naturally break apart from the rest of your endometrial lining. When the uterus contract it helps remove this obsolete layer from your uterus. This is your period.

This first day of bleeding marks Day 1 of your next cycle and it begins all over again.

Even by the second day of your period (Day 2) your body may already be building a new functional endometrial layer. 

DAY 28: Progesterone has dropped completely which triggers your period and the start of your next menstrual cycle: DAY 1

That ladies (and maybe a few gentleman) is your menstrual cycle! Hopefully this image makes a bit more sense now and you have a feel for how wonderfully complex your hormones are.

If you have any questions or want to know more about your cycle send me an email! 

Resources:

Reed, BG. (2015 May 22). The Normal Menstrual Cycle and the Control of Ovulation. https://www.ncbi.nlm.nih.gov/books/NBK279054/