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- 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.