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- [Narrator] There's actually
two different cycles occurring

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during the menstrual cycle.

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There's the ovarian cycle,
which is dealing with

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the FSH and LH hormones.

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It's dealing with how the egg's,

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the oocytes, are being matured.

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But there's also the uterine cycle.

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This is controlled by
estrogen and progesterone.

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This is the one that
actually causes the menses

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or causes the loss of
tissue from inside the body.

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So really, the menstrual
cycle is a combination

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of the ovarian and uterine cycles.

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So we start off with the ovarian cycle.

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You start with immature
follicles, so immature eggs.

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They're called primary oocytes.

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Well, at this point,
they can't do anything.

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They're just sitting there.

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So as they start to develop

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they'll start to develop
as FSH and LH increase.

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Get a little more estrogen,

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a little bit of progesterone in here.

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But as they keep developing

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they turn into a secondary oocyte.

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Now, these secondary
oocytes are more mature.

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They're getting towards
the point of ovulation.

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Eventually, a surge of LH

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is a cause

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in ovulation to occur.

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Once the ovulation occurs,

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you have some scar tissue
left over on the ovary.

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That scar tissue is
called the corpus luteum.

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It's this corpus luteum
that can actually produce

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the estrogen and progesterone.

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As these hormones, that will
control the next week of two,

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if there's no fertilization.

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Or if there is fertilization,
the corpus luteum

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can keep going for up to three months

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producing estrogen and progesterone.

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So here's a blown up view
of what an ovary looks like.

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We'll start with the immature follicles.

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Those immature follicles, really
they're just sitting there.

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The ones that are in
stasis, they're waiting.

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Then you get your primary follicle.

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For the primary follicle,
either the secondary follicle.

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Secondary follicle, is when the follicle

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finally can reach maturation.

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Upon maturation, ovulation can occur.

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And the remnant, or the
scar tissue left over,

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is the corpus luteum.

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Now, eventually though, that corpus luteum

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has to turn into an actual scar.

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So once it's done producing hormones

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it turns into the corpus luteum

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down into the corpus albicans.

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So it really just degrades into
a actual scar at this point.

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Now, the ovarian cycle does have that

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little bit of a difference I mentioned

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if fertilization does occur.

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So what's gonna end up happening is

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the tissue around the
fertilized egg, or the chorion,

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is gonna secrete hCG or
human chorionic gonadotropin.

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That particular hormone is very specific.

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So specific that that's actually what

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pregnancy tests are looking for.

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So any of the pregnancy
tests are looking to detect

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levels of hCG in the body.

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And that hCG, if it's present
because of fertilization,

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that's what keeps that
corpus luteum, that remnants

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after ovulation, going for
up to three months time.

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So it's really helping to
produce more and more estrogen.

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More and more progesterone.

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All until the placenta can take over.

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So that placenta eventually takes over

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and then it will produce the
progesterone and estrogen

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for the rest of the pregnancy.

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And the reason it's important

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to have your higher levels
of progesterone and estrogen

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during pregnancy, is
those high levels hormones

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actually inhibits further ovulation.

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So it prevents ovulation from occurring,

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therefore preventing any
possible fertilization occurring

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while a female is already pregnant.

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So one of the reasons why,

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while a female is
pregnant, she can not have

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a second or third pregnancy
start at the same time.

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Now, that's different from
having twins or triplets.

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This is not having one that,
maybe is three months along,

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and having a new pregnancy.

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That's not possible.

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All because estrogen and
progesterone are preventing

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any possible ovulation.

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So we shift into the uterine cycle now.

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The uterine cycle is
really meant to accept

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the fertilized egg.

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That's really all it's doing.

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Every month, is prepare to try
to receive a fertilized egg.

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It starts off, the first couple days,

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in the menstrual phase.

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It's usually the first four or five days.

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This varies from female to female.

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Estrogen, progesterone,
the hormones are dropping.

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And this drop in hormone

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actually causes the endometrial
lining to break down.

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That's that inside lining of the uterus

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is breaking down now.

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As it breaks down, the tissue and blood

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at once being removed, so
menstruation is occurring.

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Then you have a proliferative phase,

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this is about days five or six until about

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midway point about day 14.

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Estrogen and progesterone
start to increase again now.

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As they increase, instead
of the endometrial lining,

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that inside lining breaking down,

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with more estrogen and more progesterone

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the inside lining is
starting to build back up.

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It's getting thicker and thicker.

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It's preparing for the
possible implantation

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of a fertilized egg.

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So at day 14, ovulation.

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Does that mean that every
female ovulates on day 14, no.

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Every female has their own unique cycle.

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Some females might have
a 27 or 26 day cycle,

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some might have a 29 or 30
day cycle, 31 day cycle.

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Ovulation might be on day 12, 13, 14, 15.

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The thing is, 14 is just the average day

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on an average 28 day cycle.

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So the actual number is not important,

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it's more the mid point
throughout the cycle.

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Once ovulation occurs

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the rest of the cycle, days
give or take 15 through 28,

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is when that corpus luteum, that remnant

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that's left over on the
ovary after ovulation,

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is what's producing the hormones
progesterone and estrogen.

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The endometrial lining just
continues to build and build,

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get thicker and thicker.

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Basically what's happening,
the uterus is getting ready

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to nourish a fertilized egg.

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But will there always be a fertilized egg?

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No, for sure not.

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So if it doesn't happen, if
there is no fertilization,

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if there's no implantation of an egg,

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the cycle comes to an end
and it starts all over again.

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Right back to day one.

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Now, this chart is on the larger side.

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Don't worry, we're not trying
to go into every little detail

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but let's just look at the basic idea.

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If you look at the top,

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it's dealing with two hormones.

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Notice right in the middle how it has that

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big upsurge, that's the LH surge.

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It's that surge that causes ovulation.

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Well, after ovulation occurs

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you can look down at the third line there,

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you can see that progesterone and estrogen

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are on the higher side.

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As those hormones stay on the higher side

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it shows that the uterine
cycle, which is the bottom one,

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is thicker at that time.

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So really all it's trying to do

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is have LH surge, create an ovulation,

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increase progesterone and estrogen

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to make the uterine lining thicker.

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All in an attempt to
nourish a fertilized egg.

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Now as we start looking
at cycles of hormones.

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Throughout the entire menstrual cycle,

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the ovarian phase and the uterine phase,

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there's different hormones
that are increasing,

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decreasing at different times.

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Well, one type of cycle is referred to

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as a positive feedback.

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Well in a case of a positive feedback

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something good's going on.

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If something go is happening

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then you want it to continue to happen.

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So, in an example here, is
estrogen keeps increasing,

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will that cause the surge
in luteinizing hormone?

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That surge causes ovulation.

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So it's a positive feedback,

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more estrogen, more and more and more,

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eventually triggers LH
which triggers ovulation.

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Negative feedback,

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well that means something
wrong is going on.

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Too much of something,
too little of something,

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something's happening that shouldn't.

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A negative feedback is
meant to stop a process.

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Might be meant to reverse a process.

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Basis, meant to undo whatever went wrong.

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So in the secretory phase here,

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if you keep pretty much steady levels

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of estrogen and progesterone

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you inhibit luteinizing hormone LH and FSH

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follicle stimulating.

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So by keeping those levels there

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it prevents the other
two from being released.

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All negative feedback cycle.

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So here you can see,

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as you have the estrogen
increasing, increasing, increasing,

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eventually it hits a
point where it stimulates

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luteinizing hormone.

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Well, if you look at the
second half of the charts

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you can estrogen and
progesterone are higher levels

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that keeps LH and FSH down.