Menstrual Cycle Phases: Everything You Need To Know

Introduction to the Menstrual Cycle

Menstrual cycle is the next topic. This is also called a period, menses, or even menstruation. This is a cycle of changes that happens every month with female reproductive organs, specifically in the endometrium and the ovaries.

Onset and Duration of the Menstrual Cycle

Now, it begins at puberty. The first period is known as menarche, and it continues every single month until menopause kicks in around 50 to 55 years old. Then the periods cease to continue.

Structure and Function of the Endometrium

Now, more specifically, the endometrium is the layers of tissue that line the uterus — which I call the baby condo — since it houses the baby fetus during pregnancy. Now, the uterus lining is the endometrium and consists of two layers. The top layer is the functional layer, which is shed during monthly menstrual cycles, and underneath is the second layer, the basal layer, which aids in the feeding of the top functional layer.

Overview of the Dual Cycles

Now, the menstrual cycle itself is actually two cycles in one, working together — sort of like playing catch with the baseball, or in this case, playing catch with an egg. You see, the ovaries toss an egg over to the uterus during ovulation, and the uterus gets thick in order to catch the egg in hopes that it will get fertilized with sperm and implant into the uterine wall in order to grow a little baby fetus.

The Ovarian Cycle and Ovulation

Now, in more fancier terms, the ovarian cycle is where hormonal changes trigger the ovaries to release an egg — this is called ovulation. So just think: the ovaries release the oval-shaped egg during ovulation, kind of like tossing a ball over to the uterus.

The Uterine Cycle: The Catching Phase

And the second cycle is the uterine cycle, which I call the catching phase. The endometrium beefs up really thick to house a fertilized egg. But if the egg is not fertilized within 14 days, then it will shed each month, causing menstrual bleeding.

Cycle Phases and Mnemonics

Now that we have the basics covered, let's talk about the specifics here. The average cycle lasts around 28 days in two main phases. The first phase is the follicular phase — before an egg is released — basically before ovulation. And the luteal phase is after the egg is released — after ovulation.

Fun Analogy for Cycle Phases

So think of the F in the follicular phase as before the egg is released, and the L in the luteal phase as later, after the egg is released. So just think about the ovaries saying, “Later, homie!” as the little egg makes its way over to the uterus.

Hormonal Triggers in the Follicular Phase

Now, putting this all together and going a little bit deeper into physiology: during the ovarian cycle, the follicular phase — also called the pre-ovulatory phase — a number of hormones pop off like a party popper, starting with the hypothalamus and anterior pituitary.

The Hormone Cascade

So, the hypothalamus releases GnRH — the gonadotropin-releasing hormone — which dominoes into the release of FSH (follicle-stimulating hormone) and LH (luteinizing hormone) from the anterior pituitary.

Follicle Stimulation and Estrogen’s Role

Now, all of these hormones stimulate the follicles within the ovaries, causing ovulation, with the help of increasing amounts of estrogen — which is a steroid hormone that aids in fertilization in a variety of ways, mainly to make the endometrium really thick and turn off excessive FSH and LH hormones, acting as a negative feedback loop — kind of like turning off a light switch.

Ovulation and Start of the Luteal Phase

Now, during ovulation, the ovaries release the egg, and the endometrium gets really thick to support the new egg. This occurs from day 14 to day 28 of a 28-day cycle. Now, this is the start of the luteal phase, as the tiny egg floats away from the ovary and on its way down to the Fallopian tubes and into the uterus once again, in hopes of its ultimate goal — getting fertilized with sperm.

Role of the Corpus Luteum

So, an empty follicle within the ovary that has just released the egg now collapses and becomes a corpus luteum, or a corpus luteum, which will release both estrogen and progesterone — the sex hormones — which again help to thicken the endometrial lining in the uterus in order to help a fertilized egg implant and develop into a little baby fetus.

What Happens If Fertilization Does Not Occur

But if this egg is not fertilized within 14 days, then on day 15, there is no chance of pregnancy. Thus, there’s no need for the egg and the thick endometrial lining of the uterus. So, both of these will begin to dissolve, causing menstrual bleeding. So, we see blood and fluids exit through the cervix and the vagina, which lasts around one to eight days. And the hormones naturally reset too — so estrogen and progesterone will decrease.

The Hormonal Reset and New Cycle Start

This happens due to a collapsing corpus luteum, or corpus luteum, within the ovaries. So, this drop in estrogen and progesterone removes that negative feedback loop to the hypothalamus and the pituitary glands, and the levels of FSH begin to rise once again — and the cycle starts all over.

Fertilization and Pregnancy Hormone HCG

Now, if the egg is fertilized with sperm, then this cycle will stop as the egg attaches to the wall of the uterus. Here, now a little side note — the embryo releases HCG (human chorionic gonadotropin) — that’s a big one.

Pregnancy Tests and HCG

Now, this hormone is released during a positive pregnancy. So, that’s why we test this hormone during a pregnancy test. So, a positive test means positive for HCG hormone. This HCG helps to keep estrogen and progesterone levels high by keeping the corpus luteum, or corpus luteum, open inside the ovaries.

Birth Control and Emergency Contraception

And another little side note here — birth control, like oral contraceptives or even the ring, works by keeping estrogen and progesterone high in order to control ovulation and prevent fertilization. And the Plan B or morning-after pill is used for emergency contraception, and it works by shedding the inner lining of the uterus — the endometrium — so that the fertilized egg cannot attach and grow into a fetus and become a little baby.

Recap of Hormonal Sequence

Now, as a quick recap for all the various hormones once again:
The hypothalamus releases GnRH — the gonadotropin-releasing hormone.
The anterior pituitary releases FSH — the follicle-stimulating hormone — and LH — the luteinizing hormone.

Summary of the Cycle and Role of Estrogen and Progesterone

Now, this party popper of hormones helps to stimulate the ovarian follicles to release estrogen, which helps in ovulation — that release of the egg from the follicle. And the corpus luteum — that collapsed ovarian follicle after the egg was released — also releases estrogen and progesterone to keep the endometrial lining thick and prevent the cycle loop from starting all over again from the top.

Final Note on Fertilization and HCG Function

Now, if the egg is fertilized, it's called an embryo, and it releases HCG — that human chorionic gonadotropin — which tells the corpus luteum to keep pumping out high levels of estrogen and progesterone.

Closing and Call to Action

All right, that wraps it up. Thank you so much for watching! Don’t forget to take your quiz and download the study guides. And also feel free to share the love — share with a classmate and even your instructor.
See you guys in the next videos!