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MCAT
Course: MCAT > Unit 11
Lesson 2: Human development- Human physiological development
- Egg, sperm, and fertilization
- Early embryogenesis - Cleavage, blastulation, gastrulation, and neurulation
- Implantation
- Germ layer derivatives
- Gestation
- Major motor milestones
- Motor development
- Neonatal reflexes
- Physical development in adolescence
- Brain changes during adolescence
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Gestation
Explore the journey of pregnancy, from gestation to birth. Understand key terms like trimesters and embryogenesis. Learn how fertilization occurs around week two, leading to organogenesis and fetal development. Discover important gestational milestones, including the 50% survival rate at 24 weeks and full term at 39 weeks. Created by Jeff Otjen.
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- I can understand that there is debate or confusion about when "life" begins, but I don't understand the speaker stating atthat "conception is kind of a cloudy term and there's not really a good definition...". My dictionary defines "conception" as "the act of conceiving; the state of being conceived...fertilization; inception of pregnancy." What is "cloudy" or confusing about defining "conception" as "the time when the sperm penetrates the egg and fertilization takes place?" 0:11(3 votes)
- There wouldn't be anything "cloudy" if there were a single definition such as the one you give. The reason it is "cloudy" is that there are multiple definitions. Some define it as you do, with the sperm penetrating the egg, others at the point the genetic material fuses, and others still at the point that the blastocyst becomes implanted 8-9 days later. All of these could be considered consistent with the definition your dictionary gives.(58 votes)
- Does embryogenesis last 10 weeks (in which case it would fall on week 12) or 8 weeks (to fall on week 10)? If fertilization doesn't happen until week 2, wouldn't week 2-10 be 8 weeks? I know he says ~10, but I'd like more clarification please.(7 votes)
- From my understanding of the video the ten weeks he refers to is in reference to the LMP. Embryogenesis doesn't begin until 2 weeks after LMP and the fetus is formed 8 weeks later...or 10 weeks after LMP.(4 votes)
- Why do some pregnancies go far pass the full term?(2 votes)
- Due to many reasons, I can mention some: previous post-term pregnancies, some deficiencies in the mother, and maternal obesity(4 votes)
- organogenesis happens same time as embryogenesis?? thats what thevideo implys(3 votes)
- You can go look at the section on embryogenesis, but from my understanding, organs begin to form as part of embryogenesis, after gastrulation, with each of the germ layers giving rise to various different organs.(1 vote)
- I'm sure using week 2 as the point of fertilization is based off of ovulation, but isn't it individualized for each pregnancy?(1 vote)
- No. using week 2 as the point of fertilization is based off of the first day of your LMP. Based on ovulation the fertilization of the egg would be on week 1, day 1.(2 votes)
Video transcript
- Gestation is a term that most people use synonymously with pregnancy. Now many people use the term conception to define when a pregnancy
or gestation starts. Now, conception is kind of a cloudy term, and there's not really a good
definition for what it means. So, in general, it's probably best if we just don't use that term altogether. Now, everybody is a
little bit more clear on when gestation ends, and
that is, of course, birth. So we'd like to break this process down a little bit further. Is there an easy way that we can do that? Well, we've all been told
that gestation or pregnancy lasts about nine months. So we can draw another timeline here. Here we've got it divided
out into month-long segments, and you can see I've got one, two, three, four, five, et cetera, and you've also probably
heard the term trimester, and the trimester is a useful concept too. Now pregnancy lasts about nine months, and you can divide that
into three equal sections. So here we have a division
into the first trimester and the second trimester
and the third trimester. You can see this potentially
goes out beyond nine months. First, second, and third. But we can divvy it up more than that. In fact, it's most useful to
define it in terms of weeks. So here on the same scale I've
just given us week segments instead of monthly segments. I'll put marks at 10, 20, 30, 40 weeks. So you can see basically
how these are laid out. We have the same time frame here. The pink arrow measures
the entirety of pregnancy. The blue timeline is for months. The red timeline is trimesters, and the green timeline is for weeks. So going back to the fact that conception is not that great a term, we know that it happens somewhere here, around the beginning of pregnancy, but to be a little bit more precise, it's probably best to
define these time points. So if we're calling this week zero, we have to be able to
count from something, and the thing that most
people use to count from is the last menstrual period. This is usually something that's relatively easy to pick out, and in medical terms that's
usually abbreviated to LMP. Now, of course, that's not
when fertilization happens. Fertilization actually
happens here, at week two. And you'll remember that
fertilization is when an egg is met by a sperm and
genetic material is combined. After fertilization occurs,
we can start embryogenesis. Now embryogenesis lasts about 10 weeks, and by the end of that period, we've started as a single cell, and we've divided and divided and differentiated those cells, and we've gotten to the point where all of our organ systems are formed. So, we've undergone organogenesis. From this point on,
we're considered a fetus, and we continue to develop, but we call it fetal development. Along the way, we pass some
very important milestones. Here, at roughly 24 weeks, we hit a milestone of 50% survival, and that 50% survival is outside the womb. So if you're born at about 24 weeks, you have even odds of actually surviving, but after 24 weeks, your complication rate
significantly declines. And here we get to the
end of our 39th week, and at that point we are full term. There's a little bit of
leeway in this definition, and most people consider full term to be a range around 40 weeks, extending from 37 weeks to 42 weeks. Before that, and we're
in pre-term territory. After that, and we're
in post-term territory. Most people know that
there can be complications to being born pre-term, but there are actually also complications to being born post-term. For instance, you might be too big to fit through the birth canal. The best outcomes happen in
this range of being born term. So, there you have it. While pregnancy and gestation
is one long continuum, it can be divided into subparts
like months, trimesters, or to be probably the most scientifically accurate and useful, weeks. You start counting at
the last menstrual period at week zero. Fertilization happens at week two. You go through
embryogenesis, organogenesis, fetal development, along
with a period of growth and further development, before birth at the end of pregnancy.