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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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  • male robot hal style avatar for user Wudaifu
    I can understand that there is debate or confusion about when "life" begins, but I don't understand the speaker stating at that "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?"
    (3 votes)
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    • leafers sapling style avatar for user Peter Collingridge
      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.
      (60 votes)
  • mr pants teal style avatar for user CBaldacci
    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)
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  • blobby green style avatar for user Wuese Adyorough
    Why do some pregnancies go far pass the full term?
    (2 votes)
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  • female robot grace style avatar for user OpenMinded737
    organogenesis happens same time as embryogenesis?? thats what thevideo implys
    (3 votes)
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  • leaf green style avatar for user J P
    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)
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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.