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You were once the size of the head of a pin.
Let’s just start out with that.
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Two millimeters. At the most.
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And today -- I don’t know you, but I’m
assuming that you’re a lot larger than that.
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Many, many pin-heads from head to toe.
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Assuming the average human is about 1.7 meters,
it’s safe to say that you’re about 850
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times taller than when you started out, as
a zygote.
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I think we all deserve a round of applause.
I’m very proud of us. But how did it happen?
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How does a single cell eventually grow into
the fully-formed, several-trillion-celled
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body we’ve spent all year discussing?
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And for that matter, how the heck does a female
body nurture, protect, and generally tolerate
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growing a whole new person inside of it -- with
all the mood swings, sore and swollen parts,
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increased blood volume, constant pee breaks,
and general body weirdness?
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On an emotional level, I have no idea how the mothers
-- or the zygotes, for that matter -- endure all of that.
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But on a physiological level, like so many
aspects of our body’s functions,
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pregnancy begins and ends with the same thing:
Hormones.
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Last week we left off with a sperm and an egg, against
all odds, finally getting together to make a zygote.
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Now, how that tiny head of a pin gets to be
even a fraction of the size you are now -- a
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newborn human that’s, say, 50 centimeters
long, is complex.
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It involves a system of hormonal signals that
-- as sophisticated as they are -- get interpreted
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by the very earliest human cells into three
simple instructions:
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Divide. Differentiate. Develop.
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The first step, dividing, begins in what’s
known as the cleavage phase, when cells cleave,
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or split in two, over and over.
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It starts about 24 hours after fertilization,
when a little zygote turns from 1 cell into
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16 cells -- called blastomeres.
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The cells divide so quickly that they don’t
actually grow between divisions -- they just
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create more smaller cells.
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This allows each little cell to have more
surface area, which helps them take in the
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oxygen and nutrients they need from their
environment.
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And, of course, it also creates more raw
materials for building an even larger zygote,
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and, eventually, an embryonic human.
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Like, you don’t make a car by carving it
from a single chunk of metal. You put together
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lots of smaller components, each with its
own form and function. That’s what these
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cells are during cleavage.
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About three days after fertilization, these
divisions have formed a little berry-shaped
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cluster of cells that looks different and
complex enough to get a whole new name -- a
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morula, from the Latin for “mulberry.”
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It’s one of the cuter terms you’ll come
across in human physiology, and it also marks
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the end of the cleavage stage.
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Because: The little cells that make up the
morula don’t stay together as a solid mass
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-- instead, they start to form a hollow sphere
filled with fluid -- a blastocyst.
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A blastocyst contains a single outer layer
of large, flat trophoblast cells, and inside
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a cluster of smaller cells forms, called the
inner cell mass.
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This mass is what’s going to turn into the
embryo, eventually, while the trophoblasts
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will go on to form the placenta and blood
vessels that will nourish it.
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But, keep in mind: During all of these early
divisions, the zygote, and then the morula,
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are on the move -- they’re headed down the
fallopian tube toward the uterus.
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So, in addition to manufacturing a ton of
tiny pieces and assembling them into something
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that’s getting increasingly complex, the
whole operation is mobile. It’s kind of
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like you’re building a car while you’re
driving it.
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When the blastocyst reaches the uterus, it
just floats around for a few days, soaking
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up secretions that are full of vitamins and
glycoproteins, looking for a place to call home.
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But soon, it’s gonna need permanent nutritional
support. So, about a week after ovulation,
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it snuggles up to the endometrial layer and
starts the process of implantation.
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Now, up to this point, we’ve just been talking
about the zygote itself, and the changes it’s
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been undergoing. And we’re all very proud
of it.
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But from here on, the changes are best understood
not just in terms of the budding, soon-to-be
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embryo, but in terms of the whole environment
where its changes are taking place.
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That environment being: the mother’s body.
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Implantation is only possible, after all,
thanks to surges of estrogen and progesterone
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from the corpus luteum -- the ruptured follicle
that we talked about last time.
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And together, they prepare the endometrium
to receive the blastocyst, allowing the uterine
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lining to bind to little proteins on the trophoblasts,
holding onto them for the duration of the pregnancy.
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If all goes according to plan, implantation
takes about five days and finishes up around
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twelve days after ovulation, right around
when menstruation would otherwise kick in
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and slough off the endometrium.
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If that happened it would be bad, so the trophoblasts
secrete a luteinizing-like hormone called
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human chorionic gonadotropin, or hCG.
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This hormone bypasses the whole hypothalamic-pituitary-ovarian
axis -- if you remember that -- and talks
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directly to the corpus luteum, telling it
to keep pumping out estrogen and progesterone.
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So, the first thing to notice here is that,
from this point on, most of what happens to
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this little clump of cells is happening because:
Hormones.
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hCG is basically calling all the shots, triggering
the release of other hormones that are crucial
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to the blastocyst’s development.
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But while hormones are doing the grunt work,
you might also notice that the blastocyst
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is really in charge now. Essentially, it has
taken over hormonal control of the whole uterus.
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The release of hormones is that powerful.
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In fact, a whole new -- albeit temporary -- organ
is forming at this point, too, which is going
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to take over the hormone-controlling job -- that’s
the placenta.
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The placenta is one of the defining and most
amazing structures in the mammalian class.
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It’s an organ that only appears during pregnancy,
and is created by the melding of both maternal
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and embryonic tissues.
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Together with the umbilical cord, it provides
for the direct transfer of nutrients, hormones,
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and wastes between mother and offspring.
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These elements start to really take shape
after implantation is complete, and we enter
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the embryonic stage. This is where the blastocyst
differentiates into various cell types and
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develops into a legit embryo, surrounded by
an amniotic sac, and hooked up to the placenta.
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And around the end of week eight, this tiny
thing is now officially a fetus.
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Over the next several months, it rapidly develops
organ systems and bones, and grows into what
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you see in the delivery room.
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But rather than focus on those changes, I
want to look more at what profound, and frankly
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sometimes bizarre, adaptations the mother
is going through.
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The most obvious changes are of course the
anatomical changes.
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Basically, everything’s getting huge.
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Her breasts swell and engorge with blood,
and her baby bump gains dimension as the normally
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fist-sized uterus expands, pushing organs
out of the way until it pretty much takes
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up the entire abdominal cavity from the diaphragm
to the bladder.
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The growing placenta is still secreting estrogen
and progesterone, but it’s also pumping
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out relaxin, a hormone that loosens joints
and ligaments to increase flexibility, and
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it also releases human placental lactogen,
or hPL.
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This hormone cocktail does helpful stuff like
tell the fetus to grow, and get the breasts
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ready to lactate, and it also causes the mother’s
body to start hoarding glucose for the fetus to use.
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This increase in metabolism, combined with
the fact that the kidneys also have to process
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waste from the fetus, leads to greater urine production,
which every expectant mother is familiar with.
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It also has a huge effect on the cardiovascular
system, because a pregnant woman’s blood
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volume can increase by as much as 40 percent.
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40 PERCENT!
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Imagine walking around with an extra 2 liter
bottle’s worth of blood in your body, and
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how much harder your heart would have to work
to move it all around.
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This increased blood flow and pressure can
actually make your gums swell and bleed, and
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fluid retention can literally change the shape
of your corneas, potentially blurring your vision.
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Not only that, but the expanded uterus compresses
pelvic blood vessels, affecting veins’ ability
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to bring blood up from the lower limbs, resulting
in swelling, varicose veins, and if all that
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weren’t bad enough, hemorrhoids.
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Fortunately, every pregnancy eventually comes
to an end, usually about 38 to 40 weeks after
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fertilization, if all goes as planned.
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But unfortunately, the process of getting
the baby out is pretty much a painful mess.
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Ask any mother -- giving birth is not for
the faint of heart.
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The process of preparing the body for labor,
and then actually initiating it, begins -- AGAIN!
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-- with a change in hormones.
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Up until the last few weeks before birth,
the placenta has been kicking out equally
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high amounts of both progesterone and estrogen.
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One of progesterone’s main jobs has been
to keep the smooth muscles in the uterus relaxed,
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so they can’t contract and stimulate labor
too early.
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But as the due date nears, the mother undergoes
a sudden decline in progesterone.
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Now, estrogen takes over. This is partially
because the fetus itself is ready to go, and
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it starts releasing hormones like cortisol,
that tell the placenta to release even more
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estrogen to get the uterus ready for birth.
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Just like hCG calls the shots around the
time of implantation, here estrogen is
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barking out all kinds of orders.
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For one thing, it prepares the uterus to start
receiving new chemical signals, by triggering
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its myometrial cells to start making receptors
for the hormone oxytocin.
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It also triggers the formation of gap junctions
between smooth muscle cells in the uterus
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-- this will let individual muscle cells contract
simultaneously when the time comes.
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Then, as labor nears, special cells in the
fetus itself start secreting oxytocin, which
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binds to all the newly-minted receptors and
tells the placenta to release prostaglandins.
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Together, both of these hormones -- oxytocin
and prostaglandins -- stimulate the uterine
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muscles to start contracting.
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When the contractions get strong enough to
distend the cervix, it stimulates the release
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of even more oxytocin and prostaglandins,
which keep the contractions rolling in one
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big positive feedback loop, initiating labor.
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The earliest stage of labor, dilation, is
the period from when the first contractions
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begin, to when the cervix becomes fully dilated,
to about 10 centimeters.
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During this time, each new contraction pushes
the infant’s head against the cervix, causing
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the cervix to thin and dilate.
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Once the cervix is fully dilated, the mother
should feel urge to push. The resulting expulsion
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stage lasts from full dilation through crowning
and actual delivery, as the infant is pushed
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head-first through the cervix and out the
vagina.
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But even after the baby is out, the mother
still has a placenta to get out. Within about
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30 minutes of delivery, strong contractions
carry out the placental stage of labor, dislodging
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the placenta from the uterine wall to deliver
the so-called afterbirth.
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Then, finally, the mother can rest, and marvel
at how her body just built another tiny body
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inside it -- complete with all the complex
systems we’ve spent the last year talking about.
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And maybe someday that little baby will grow
up and get a twinkle in its eye, and combine
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alleles with somebody else, and start the
whole process over again.
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And that is how the human race continues to
exist.
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Today you learned about the stages of pregnancy,
beginning with how a zygote develops into
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blastomeres to a morula to a blastocyst and
finally to an embryo and a fetus. You also
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learned about the amazing anatomical changes
that take place in the mother, and the hormonal
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sequence of events that lead to labor. After
that, all you have to worry about is how you’re
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gonna put the little thing through college.
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Thank you to our Headmaster of Learning, Linnea
Boyev, and thanks to all of our Patreon patrons
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whose monthly contributions help make Crash
Course possible, not only for themselves,
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but for everyone, everywhere. If you like
Crash Course and want to help us keep making
00:10:04
videos like this one, you can go to patreon.com/crashcourse.
00:10:07
This episode was filmed in the Doctor Cheryl
C. Kinney Crash Course Studio, it was written
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by Kathleen Yale, the script was edited by
Blake de Pastino, and our consultant is Dr.
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Brandon Jackson. It was directed by Nicholas
Jenkins, edited by Nicole Sweeney, our sound
00:10:19
designer is Michael Aranda, and the Graphics
team is Thought Cafe.
