From Zygote to Fetus

CrashCourse

Feb 25, 2026

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3 min read

YouTube video ID: BtsSbZ85yiQ

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Size at conception: roughly the size of a pinhead (≈ 2 mm).
Growth factor: an average adult (≈ 1.7 m) is about 850 times taller than the original zygote.

Early Cell Divisions

Cleavage phase – begins ~24 h after fertilization.
The single‑cell zygote divides repeatedly without growing, producing 16 small cells called blastomeres.
Rapid division increases surface area for oxygen and nutrient uptake.
Morula formation – ~3 days post‑fertilization.
Blastomeres compact into a berry‑shaped cluster called a morula (Latin for “mulberry”).
Blastocyst formation – follows the morula stage.
Cells reorganize into a hollow sphere filled with fluid.
Outer layer: large, flat trophoblast cells (future placenta).
Inner cell mass: cluster that will become the embryo.

Implantation

The blastocyst travels down the fallopian tube to the uterus, floating for several days while absorbing vitamins and glycoproteins.
About 1 week after ovulation, it attaches to the endometrial lining (implantation).

Hormonal context
- Estrogen and progesterone from the corpus luteum prepare the endometrium for implantation.
- After implantation, the trophoblast secretes human chorionic gonadotropin (hCG), which signals the corpus luteum to continue producing estrogen and progesterone, bypassing the hypothalamic‑pituitary‑ovarian axis.

Placenta Development

The trophoblast and maternal tissues merge to form the placenta, a temporary organ that mediates nutrient, hormone, and waste exchange between mother and fetus.
The placenta also begins producing relaxin (loosens joints) and human placental lactogen (hPL) (promotes fetal growth and maternal lactation preparation).

Embryonic to Fetal Transition

After implantation, the embryo differentiates into various cell types within an amniotic sac and connects to the placenta.
By the end of week 8, the embryo is termed a fetus and continues rapid organ and skeletal development.

Maternal Physiological Adaptations

Anatomical changes:
Breasts engorge with blood.
Uterus expands from fist‑size to fill most of the abdominal cavity, displacing other organs.
Cardiovascular:
Blood volume increases up to 40 % (≈ 2 L extra blood).
Increased volume raises cardiac workload, can cause swollen gums, altered corneal shape, and blurred vision.
Renal:
Greater waste processing leads to increased urine production.
Hormonal effects:
Relaxin loosens ligaments for flexibility.
hPL drives maternal glucose storage for fetal use and stimulates breast development.
Vascular:
Expanded uterus compresses pelvic veins → swelling, varicose veins, hemorrhoids.

Hormonal Cascade Initiating Labor

Progesterone decline – Near term, maternal progesterone drops, removing its relaxing effect on uterine smooth muscle.
Estrogen rise – Fetal cortisol prompts the placenta to release more estrogen, which:
Induces uterine myometrial cells to produce oxytocin receptors.
Promotes formation of gap junctions for coordinated muscle contractions.
Fetal oxytocin release – Binds to newly formed receptors, stimulating the placenta to secrete prostaglandins.
Positive feedback loop – Oxytocin and prostaglandins together cause uterine contractions that:
Dilate the cervix (up to ~10 cm).
Trigger further oxytocin and prostaglandin release, intensifying contractions.

Stages of Labor

Dilation (first stage): From onset of regular contractions to full cervical dilation (~10 cm).
Expulsion (second stage): Mother feels the urge to push; the infant’s head descends, the cervix thins, and the baby is delivered head‑first.
Placental stage (third stage): Within ~30 minutes post‑delivery, strong uterine contractions expel the placenta ("afterbirth").

Summary

Human development proceeds from a pin‑head‑sized zygote through cleavage, morula, blastocyst, embryo, and fetus, driven by tightly regulated hormonal signals.
The mother’s body undergoes extensive anatomical, cardiovascular, renal, and hormonal adaptations to support the growing fetus.
Labor is initiated by a coordinated hormonal shift—declining progesterum, rising estrogen, fetal oxytocin, and prostaglandins—culminating in uterine contractions that deliver the baby and placenta.

Takeaways

The zygote starts at roughly 2 mm and grows through stages including cleavage, morula, blastocyst, embryo, and fetus, reaching a size about 850 times larger than at conception by week 8.
Implantation occurs about one week after ovulation and is supported by estrogen, progesterone, and hCG that maintain the corpus luteum.
The placenta forms from trophoblast and maternal tissue, mediating nutrient and waste exchange while producing hormones such as relaxin and hPL.
Maternal adaptations include increased blood volume, cardiovascular workload, renal changes, and hormonal effects that prepare the body for pregnancy.
Labor is triggered by a hormonal cascade involving progesterone decline, estrogen rise, fetal oxytocin, and prostaglandins, leading to uterine contractions and cervical dilation.
The three stages of labor—dilation, expulsion, and placental delivery—complete the birth process.

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Helpful resources related to this video

If you want to practice or explore the concepts discussed in the video, these commonly used tools may help.

Embryology Textbook Recommended

Provides detailed visual and textual explanations of early developmental stages, supporting study of zygote to fetal transition

Amazon →

Pregnancy Guide

Offers practical information on maternal physiological changes and labor preparation, useful for expecting mothers

Amazon →

Prenatal Nutrition Book

Explains nutritional needs that support placental function and fetal growth, relevant to hormonal and physiological adaptations

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Full Transcript YouTube

You were once the size of the head of a pin.
Let’s just start out with that.
Two millimeters. At the most.
And today -- I don’t know you, but I’m
assuming that you’re a lot larger than that.
Many, many pin-heads from head to toe.
Assuming the average human is about 1.7 meters,
it’s safe to say that you’re about 850
times taller than when you started out, as
a zygote.
I think we all deserve a round of applause.
I’m very proud of us. But how did it happen?
How does a single cell eventually grow into
the fully-formed, several-trillion-celled
body we’ve spent all year discussing?
And for that matter, how the heck does a female
body nurture, protect, and generally tolerate
growing a whole new person inside of it -- with
all the mood swings, sore and swollen parts,
increased blood volume, constant pee breaks,
and general body weirdness?
On an emotional level, I have no idea how the mothers
-- or the zygotes, for that matter -- endure all of that.
But on a physiological level, like so many
aspects of our body’s functions,
pregnancy begins and ends with the same thing:
Hormones.
Last week we left off with a sperm and an egg, against
all odds, finally getting together to make a zygote.
Now, how that tiny head of a pin gets to be
even a fraction of the size you are now -- a
newborn human that’s, say, 50 centimeters
long, is complex.
It involves a system of hormonal signals that
-- as sophisticated as they are -- get interpreted
by the very earliest human cells into three
simple instructions:
Divide. Differentiate. Develop.
The first step, dividing, begins in what’s
known as the cleavage phase, when cells cleave,
or split in two, over and over.
It starts about 24 hours after fertilization,
when a little zygote turns from 1 cell into
16 cells -- called blastomeres.
The cells divide so quickly that they don’t
actually grow between divisions -- they just
create more smaller cells.
This allows each little cell to have more
surface area, which helps them take in the
oxygen and nutrients they need from their
environment.
And, of course, it also creates more raw
materials for building an even larger zygote,
and, eventually, an embryonic human.
Like, you don’t make a car by carving it
from a single chunk of metal. You put together
lots of smaller components, each with its
own form and function. That’s what these
cells are during cleavage.
About three days after fertilization, these
divisions have formed a little berry-shaped
cluster of cells that looks different and
complex enough to get a whole new name -- a
morula, from the Latin for “mulberry.”
It’s one of the cuter terms you’ll come
across in human physiology, and it also marks
the end of the cleavage stage.
Because: The little cells that make up the
morula don’t stay together as a solid mass
-- instead, they start to form a hollow sphere
filled with fluid -- a blastocyst.
A blastocyst contains a single outer layer
of large, flat trophoblast cells, and inside
a cluster of smaller cells forms, called the
inner cell mass.
This mass is what’s going to turn into the
embryo, eventually, while the trophoblasts
will go on to form the placenta and blood
vessels that will nourish it.
But, keep in mind: During all of these early
divisions, the zygote, and then the morula,
are on the move -- they’re headed down the
fallopian tube toward the uterus.
So, in addition to manufacturing a ton of
tiny pieces and assembling them into something
that’s getting increasingly complex, the
whole operation is mobile. It’s kind of
like you’re building a car while you’re
driving it.
When the blastocyst reaches the uterus, it
just floats around for a few days, soaking
up secretions that are full of vitamins and
glycoproteins, looking for a place to call home.
But soon, it’s gonna need permanent nutritional
support. So, about a week after ovulation,
it snuggles up to the endometrial layer and
starts the process of implantation.
Now, up to this point, we’ve just been talking
about the zygote itself, and the changes it’s
been undergoing. And we’re all very proud
of it.
But from here on, the changes are best understood
not just in terms of the budding, soon-to-be
embryo, but in terms of the whole environment
where its changes are taking place.
That environment being: the mother’s body.
Implantation is only possible, after all,
thanks to surges of estrogen and progesterone
from the corpus luteum -- the ruptured follicle
that we talked about last time.
And together, they prepare the endometrium
to receive the blastocyst, allowing the uterine
lining to bind to little proteins on the trophoblasts,
holding onto them for the duration of the pregnancy.
If all goes according to plan, implantation
takes about five days and finishes up around
twelve days after ovulation, right around
when menstruation would otherwise kick in
and slough off the endometrium.
If that happened it would be bad, so the trophoblasts
secrete a luteinizing-like hormone called
human chorionic gonadotropin, or hCG.
This hormone bypasses the whole hypothalamic-pituitary-ovarian
axis -- if you remember that -- and talks
directly to the corpus luteum, telling it
to keep pumping out estrogen and progesterone.
So, the first thing to notice here is that,
from this point on, most of what happens to
this little clump of cells is happening because:
Hormones.
hCG is basically calling all the shots, triggering
the release of other hormones that are crucial
to the blastocyst’s development.
But while hormones are doing the grunt work,
you might also notice that the blastocyst
is really in charge now. Essentially, it has
taken over hormonal control of the whole uterus.
The release of hormones is that powerful.
In fact, a whole new -- albeit temporary -- organ
is forming at this point, too, which is going
to take over the hormone-controlling job -- that’s
the placenta.
The placenta is one of the defining and most
amazing structures in the mammalian class.
It’s an organ that only appears during pregnancy,
and is created by the melding of both maternal
and embryonic tissues.
Together with the umbilical cord, it provides
for the direct transfer of nutrients, hormones,
and wastes between mother and offspring.
These elements start to really take shape
after implantation is complete, and we enter
the embryonic stage. This is where the blastocyst
differentiates into various cell types and
develops into a legit embryo, surrounded by
an amniotic sac, and hooked up to the placenta.
And around the end of week eight, this tiny
thing is now officially a fetus.
Over the next several months, it rapidly develops
organ systems and bones, and grows into what
you see in the delivery room.
But rather than focus on those changes, I
want to look more at what profound, and frankly
sometimes bizarre, adaptations the mother
is going through.
The most obvious changes are of course the
anatomical changes.
Basically, everything’s getting huge.
Her breasts swell and engorge with blood,
and her baby bump gains dimension as the normally
fist-sized uterus expands, pushing organs
out of the way until it pretty much takes
up the entire abdominal cavity from the diaphragm
to the bladder.
The growing placenta is still secreting estrogen
and progesterone, but it’s also pumping
out relaxin, a hormone that loosens joints
and ligaments to increase flexibility, and
it also releases human placental lactogen,
or hPL.
This hormone cocktail does helpful stuff like
tell the fetus to grow, and get the breasts
ready to lactate, and it also causes the mother’s
body to start hoarding glucose for the fetus to use.
This increase in metabolism, combined with
the fact that the kidneys also have to process
waste from the fetus, leads to greater urine production,
which every expectant mother is familiar with.
It also has a huge effect on the cardiovascular
system, because a pregnant woman’s blood
volume can increase by as much as 40 percent.
40 PERCENT!
Imagine walking around with an extra 2 liter
bottle’s worth of blood in your body, and
how much harder your heart would have to work
to move it all around.
This increased blood flow and pressure can
actually make your gums swell and bleed, and
fluid retention can literally change the shape
of your corneas, potentially blurring your vision.
Not only that, but the expanded uterus compresses
pelvic blood vessels, affecting veins’ ability
to bring blood up from the lower limbs, resulting
in swelling, varicose veins, and if all that
weren’t bad enough, hemorrhoids.
Fortunately, every pregnancy eventually comes
to an end, usually about 38 to 40 weeks after
fertilization, if all goes as planned.
But unfortunately, the process of getting
the baby out is pretty much a painful mess.
Ask any mother -- giving birth is not for
the faint of heart.
The process of preparing the body for labor,
and then actually initiating it, begins -- AGAIN!
-- with a change in hormones.
Up until the last few weeks before birth,
the placenta has been kicking out equally
high amounts of both progesterone and estrogen.
One of progesterone’s main jobs has been
to keep the smooth muscles in the uterus relaxed,
so they can’t contract and stimulate labor
too early.
But as the due date nears, the mother undergoes
a sudden decline in progesterone.
Now, estrogen takes over. This is partially
because the fetus itself is ready to go, and
it starts releasing hormones like cortisol,
that tell the placenta to release even more
estrogen to get the uterus ready for birth.
Just like hCG calls the shots around the
time of implantation, here estrogen is
barking out all kinds of orders.
For one thing, it prepares the uterus to start
receiving new chemical signals, by triggering
its myometrial cells to start making receptors
for the hormone oxytocin.
It also triggers the formation of gap junctions
between smooth muscle cells in the uterus
-- this will let individual muscle cells contract
simultaneously when the time comes.
Then, as labor nears, special cells in the
fetus itself start secreting oxytocin, which
binds to all the newly-minted receptors and
tells the placenta to release prostaglandins.
Together, both of these hormones -- oxytocin
and prostaglandins -- stimulate the uterine
muscles to start contracting.
When the contractions get strong enough to
distend the cervix, it stimulates the release
of even more oxytocin and prostaglandins,
which keep the contractions rolling in one
big positive feedback loop, initiating labor.
The earliest stage of labor, dilation, is
the period from when the first contractions
begin, to when the cervix becomes fully dilated,
to about 10 centimeters.
During this time, each new contraction pushes
the infant’s head against the cervix, causing
the cervix to thin and dilate.
Once the cervix is fully dilated, the mother
should feel urge to push. The resulting expulsion
stage lasts from full dilation through crowning
and actual delivery, as the infant is pushed
head-first through the cervix and out the
vagina.
But even after the baby is out, the mother
still has a placenta to get out. Within about
30 minutes of delivery, strong contractions
carry out the placental stage of labor, dislodging
the placenta from the uterine wall to deliver
the so-called afterbirth.
Then, finally, the mother can rest, and marvel
at how her body just built another tiny body
inside it -- complete with all the complex
systems we’ve spent the last year talking about.
And maybe someday that little baby will grow
up and get a twinkle in its eye, and combine
alleles with somebody else, and start the
whole process over again.
And that is how the human race continues to
exist.
Today you learned about the stages of pregnancy,
beginning with how a zygote develops into
blastomeres to a morula to a blastocyst and
finally to an embryo and a fetus. You also
learned about the amazing anatomical changes
that take place in the mother, and the hormonal
sequence of events that lead to labor. After
that, all you have to worry about is how you’re
gonna put the little thing through college.
Thank you to our Headmaster of Learning, Linnea
Boyev, and thanks to all of our Patreon patrons
whose monthly contributions help make Crash
Course possible, not only for themselves,
but for everyone, everywhere. If you like
Crash Course and want to help us keep making
videos like this one, you can go to patreon.com/crashcourse.
This episode was filmed in the Doctor Cheryl
C. Kinney Crash Course Studio, it was written
by Kathleen Yale, the script was edited by
Blake de Pastino, and our consultant is Dr.
Brandon Jackson. It was directed by Nicholas
Jenkins, edited by Nicole Sweeney, our sound
designer is Michael Aranda, and the Graphics
team is Thought Cafe.