In this video I discuss prenatal development from conception until birth. This can be divided into the germinal stage, embryonic stage, and fetal stage. Conception occurs when a sperm cell and egg cell join to create a zygote, which then travels down the fallopian tube and attaches to the uterus. This then develops into an embryo with a heart and the beginnings of a body, then a fetus as it develops a skeleton and muscles. The womb environment can have important influences on development, particularly in the case of the negative effects of teratogens like lead, mercury, viruses, or drugs. Learning also begins to occur before birth, such as a fetus learning to recognize the sound of the mother’s voice.
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Video Transcript
Hi, I’m Michael Corayer and this is a Psych Exam Review. In this video we’re going to look at prenatal development or development that occurs before birth.
We’re going to start at the very beginning with conception and this is the joining of a sperm cell, which is the smallest single human cell, and an egg cell, which is the largest single human cell, and these joined together to form what’s called a zygote. And the zygote contains chromosomes from the sperm and the egg. Now, which chromosomes it contains is up to chance; this is the genetic lottery. So the egg will contain an assortment of chromosomes from the mother and the sperm will contain an assortment of chromosomes from the father and these will be joined together to give you the 23 pairs of chromosomes that make you, you.
Now you might wonder, which sperm cell manages to fertilize the egg? Well, many of the about 200 million sperm cells that will be released will be defective; they won’t be capable of swimming properly and so these are out of the running. Now we also have a problem where there’s two fallopian tubes but only one of these will have an egg waiting to be fertilized at the end of it. So many of the sperm cells will choose the incorrect fallopian tube. And then of the ones that choose the correct tube, only about 200 will get close to the egg and of these just one will successfully fertilize it. Once this happens, the egg will release an enzyme that will prevent other sperm cells from fertilizing it and once this happens we have this zygote.
And we now enter the germinal stage which lasts for about the first two weeks after conception. And during this stage the zygote has to travel down the fallopian tube and reach the uterus where it can attach itself to the uterine walls. And this process is not guaranteed to happen successfully, about half the time this process will fail. The zygote will not successfully attach itself to the walls of the uterus and that will be the end of the story.
But if this happens successfully, then we can move to the next stage of development, which is the embryonic stage. And the zygote is no longer referred to as a zygote, it’s now an embryo. And this stage will last from about the second to the eighth weeks of the pregnancy and this embryo is only about an inch long at the beginning of this stage but it already has a beating heart and the beginnings of arms and legs. Now up until this point the development has been the same for male and females. What I mean by this is, it doesn’t matter if you have the chromosomes for being male or the chromosomes for being female, the development is identical. But during this embryonic stage we’ll begin to switch this and this happens through the production of androgens or male sex hormones. And so it’s sometimes described as being female as the “default state” meaning without the presence of androgens any embryo will develop to become female. But if we have the production of androgens then this will turn the embryo male and it will begin developing male characteristics.
Now beginning in about the ninth week we entered what’s called the fetal stage. At this point the embryo is now referred to as a fetus and this lasts from about the ninth week up until birth. And the fetus is beginning to develop a skeleton and muscles and an interesting thing about human development is that at birth our brain is only about 25% of its eventual adult size. So we’re very underdeveloped when we’re born and we’re very vulnerable and we have a lot of development left to do. This is quite different from many other species. A lot of other species are born pretty much fully formed but we have a lot of growing left to do. Our brain won’t be fully developed until about the age of 25 and this opens the possibility that we can have greater influence of learning and the environment on our development, right? Because so much of our development is happening after birth and this means that the environment can have a stronger impact on how we develop.
And we should also remember, when we think about environmental influence on development, that the womb is also considered environment. I’ve mentioned this in previous videos but when we think about the nature-nurture debate or genes and environment and how they interact, we have to remember that the womb is part of the environment. And so one way we see this is through the placenta, which is the organ that links the bloodstreams of the mother and the child. And so things that are in the mother’s bloodstream can get into the bloodstream of the child and some of these things can harm development. These are called teratogens, and teratogen is Latin for “monster maker”. This is anything that can harm the development of the offspring. And so this could be a chemical like lead or mercury that finds its way into the mothers bloodstream, or it could be something like a virus the mother gets infected with, the virus, that virus could then infect the fetus that’s developing. Or it could be something that the mother’s doing, something like using tobacco. So the nicotine gets into the bloodstream and this could then influence development. In the case of mothers who smoke during their pregnancy, they’re more likely to have children with low birth weight and the children are more likely to have attention and perception problems.
Another teratogen is alcohol and so a mother who drinks heavily during pregnancy is likely to have a child with what’s called “fetal alcohol syndrome” and this is associated with intellectual disability. And it also has certain physical characteristics associated with it with which are influenced during this period of development. So you can see that children with fetal alcohol syndrome are likely to have smaller eye openings and another characteristic physical feature of this is a smooth philtrum. So the philtrum is the ridge on your upper lip and children with fetal alcohol syndrome tend to have a smooth upper lip; they don’t have this ridge.
Now, it’s also the case that the fetus influences the mother. Now, of course, this is fairly obvious during pregnancy. We see a lot of physical changes in the mother but it’s also the case that the fetus may continue influencing the mother even after birth because cells from the child actually remain in the mother’s body for years or potentially even decades following birth. This is referred to as “fetal microchimerism” and as we think about the womb as environment, we should also remember that this means that learning can occur in the womb. This is demonstrated in a study by Anthony DeCasper and Melanie Spence from 1986. What DeCasper and Spence did was they had pregnant mothers read The Cat in the Hat, so they read this story to their fetuses twice a day and then following birth the infants showed a preference for the sound of the mother’s voice reading that story versus a stranger’s voice reading the same story. This meant that these very, very young infants were already able to recognize the sound of their mother’s voice and this learning of what the mother’s voice sounded like must have occurred in the womb before birth.
And when we get to the idea of birth happening this brings us now from a fetus to an infant, so we enter the infancy stage and this is what we’ll look at in more detail in the next video. I hope you found this helpful, if so, please like the video and subscribe to the channel for more. Thanks for watching!
