 In this video we are going to discuss about the events that occur after fertilization has happened in humans. So you start off as a single celled zygote within your mother's fallopian tube after fertilization between the sperm and egg cell has occurred. The zygote keeps dividing and as it divides it moves through the fallopian tube and reaches the uterus. When it has reached the uterus, the zygote is now in its blastocyst stage with its outer layer called the trophoblast and an inner cell mass. It is the blastocyst that gets implanted within the endometrial lining of the uterus. This whole process takes around 8-9 days after fertilization has occurred. Immediately after the blastocyst is implanted, the inner cell mass begins to divide and forms the three germ layers. The outer ectoderm, the middle mesoderm and the inner endoderm. It is from these three layers that all cells and tissues in the body are produced. Take a look at the different tissues the different layers give rise to. The development of the baby inside the uterus spans around 40 weeks. So the catch here is that week zero begins when the mother had her last menstrual period. Two weeks after the menstrual period is when fertilization is believed to occur because that's when ovulation occurs. So whatever calculation is done during this period starts from the mother's last menstrual period and that is week zero. So the development of the baby inside the uterus spans three trimesters and each trimester is around 3 months. And this development during the three trimesters, this pregnancy development is called gestation. Once the three germ layers form, they keep dividing and the blastocyst can now be called an embryo. The embryological stage lasts around 8 weeks. Take a look at a human embryo at around 8 to 9 weeks of time. The early embryo begins to have basic human characteristics like the head and eyes. And these bud-like structures here, they give rise to the limbs later on. After about 9 weeks, the embryo can now be called a fetus. The fetal stage lasts till 40 weeks till when the fetus or baby is born. During the first trimester, all organs like the heart and lungs begin to form. Hands and feet that is the limbs also begin to develop. During this time, the placenta also develops. The placenta serves as a link between the mother and the baby and provides the baby with nutrients and oxygen. We'll learn more about placenta in just a while. During the second trimester, the organs are more developed. Hands and legs that is limbs are more developed as well. And towards the end of the second trimester which is around 26 to 27 weeks, movements of the baby can be felt as well. At the end of the third trimester, finishing touches are added to the developing organs. Lungs are fully developed but they are non-functional till the baby is born. Let's discuss a bit more about placenta. When fully developed, the placenta is about 9 to 10 inches in diameter around the size of a dinner plate. It is formed from the trophoblast, that is the trophoblast of the blastocyst. After the blastocyst gets implanted. So the placenta has two sides, the fetal side and the maternal side. So the fetal side has this structure called the chorionic plate and it is formed from the trophoblast. The basal plate is the maternal side of the placenta and it is attached to the uterine wall. This is the uterus and this basal plate is part of the uterine wall. So what happens after implantation is the trophoblast begins to divide and forms these projections called chorionic villi. And this image actually just shows one projection like this but there are several projections that begin to form from the trophoblast. Within these projections are umbilical arteries and umbilical veins. So these capillaries that you see here, they join to form the two umbilical arteries and one umbilical vein. And this space here is sort of like an empty space into which uterine arteries pump blood into. So the uterine artery is the artery that provides blood to the uterus. So the uterine artery pumps blood into this space and this blood is filled with oxygen and nutrients. So what happens is oxygen and nutrients diffuse into these villi, diffuse into these projections and they are picked up by the umbilical vein. And the umbilical vein transports the oxygen and nutrient rich blood to the fetus. From the fetus umbilical arteries bring in oxygen depleted or carbon dioxide and waste rich blood. And this carbon dioxide and waste also gets diffused into this space and that is picked up by the uterine vein. So the placenta sort of acts like a barrier between the fetal circulation and the maternal circulation. It doesn't allow both circulations to directly come in contact. It serves as an interface between the mother and the developing fetus. Apart from providing the fetus with nutrients, the placenta also acts as an endocrine organ and produces several hormones like progesterone, human-choreonic-gernotropin and relaxing. So these hormones play an important role in maintaining pregnancy. For example, progesterone is needed to maintain the endometrial lining of the uterus. It is very crucial for that. HCG is excreted in the urine and it is what is tested by the home pregnancy kits. Apart from these hormones, other hormones are also produced in more quantities during pregnancies in the mother such as thyroid syn, which is needed to compensate for the increased metabolism of the mother. So these hormones cause changes in the maternal body during pregnancy. Some of these changes are listed here. So the uterus increases in size to help accommodate the growing baby. The breast size increases because mammary glands are developing during pregnancy and these mammary glands produce milk after the baby is born, which provides the baby with nutrients. The heart rate and oxygen consumption of the mother also increases because more oxygen and more blood is needed to supply the baby. When the baby is fully developed, uterine contractions start forming and these contractions will help deliver the baby.