Prenatal Development
Prenatal development is the period of development from conception through birth. The whole period is divided into 3 trimesters. In the first trimester, the developing human is a zygote, an embryo and the fetus. In the second trimester, the developing human is still a fetus and by the middle of the trimester, the fetus would have grown large enough that the mother will be able to feel the movements. By the third trimester, the baby is already formed and has more chances of survival with extra support if born prematurely.
First Trimester:
Month 1:Zygote:
The zygote is one celled which during the first week multiplies and forms the blastocyst. The first cell duplication is long and takes about 30 hours from conception. After that new cells are added at a faster rate. By 4 days, there are already 60 to 70 cells which form a ball called blastocyst. The blastocyst is of 2 layers: the inner layer is called embryonic disk which will become the new organism and the outer ring of cells is called trophoblast which will provide protective covering and nourishment.
In the second week, the blastocyst burrows into the uterine lining. With the help of the nourishing blood, it grows fast. The trophoblast multiplies first. It develops a membrane called amnion that keeps the temperature in the womb constant and cushions against any jerks caused by the mother's movements. A yolk sac is also formed, which produces blood cells till liver, spleen and bone marrow are developed enough to do it.
These events are very uncertain. Almost 30 percent of the zygotes do not make up to this stage. By the end of the second week, the cells of the trophoblast form another protective membrane called the chorion surrounding the amnion. Tiny hairlike blood vessels emerge from the chorion and burrow into the uterine wall. This is when the placenta starts to develop. The placenta permits food and oxygen to reach the organism and waste products to be carried away. The membrane allows these substances to be exchanged but prevents the mother's and embryo's blood from mixing directly.
The placenta is connected with the developing human by an umbilical cord. By the course of pregnancy, the umbilical cord grows to a length of 1 to 3 feet. The cord contains one large vein that brings in blood loaded with nutrients and 2 arteries that remove waste products.
All these dramatic changes take place before most mothers are aware of the pregnancy.
Embryo:
The period of embryo lasts roughly from the third to eighth week. The foundation for all body structures and internal organs are laid down during this period. Hence the changes are rapid. During the third week of pregnancy, the embryonic disk forms three layers of cells. The ectoderm, will become the nervous system and skin, the mesoderm which becomes muscles, skeleton, circulatory system and other internal organs and the endoderm which forms into digestive system, lungs, urinary tract and glands.
The nervous system is the first to form. The ectoderm folds to form a neural tube. The top of this tube swells to become the brain in 3½ weeks. Tiny neurons are formed in this tube, and they travel to their own locations where they will form important parts of brain.
Once the nervous system is developing, the heart begins to pump blood and muscles, backbones, ribs and digestive tract are formed. By the end of the first month, the embryo is just ¼ inch long and consists of millions of cells with specific function.
Month 2:In the second month, embryo becomes 1 inch long and weighs one-seventh of an ounce. Ears, nose, jaws, eyes and neck will form. Small buds can be detected in the place of arms, legs and even fingers and toes. Intestines grow, heart develops separate chambers. Liver and spleen are developed enough to produce blood cells. The posture is more upright. The embryo now responds to touch especially near mouth and feet area. It can move but the movement is too small to be felt by mother.
Fetus:
Month 3:In the third month, nervous system, organs and muscles start to get organized and connected. The fetus kicks, bends its arms, curls fingers and toes, opens mouth and even sucks its thumb. Heart beat is strong so it could be heard with a stethoscope. Lungs begin to contract and expand. Finger nails, tooth buds are formed. Eyes open and close because eye lids are also formed. The sex of the fetus can be detected as the external genitals are well formed by the end of the third month.
Second Trimester:
Month 4:Lower parts of the body begin to develop at a faster rate. So the head which was about ½ of the whole body becomes only ¼th of the body. Skin is thick red in color showing the absence of fat and the flow of blood.
Month 5:During the fifth month, the fetus is large enough for the mother to feel its movements. The fetus is completely covered with a substance called vernix, which protects the skin from chapping due to the long months spent in the amniotic fluid. Also a white hair called lanugo covers the body to help vernix stick to the skin.
Month 6:By the sixth month, many organs of the fetus are well developed. Most of the brain neurons are in place though some are yet to be produced. There are some cells names glial cells which support and feed the neurons. These cells continue to grow rapidly through out pregnancy and even after birth. The fetus would have got new behavioral capabilities by this time. A fetus after 5 months can be stimulated or irritated by sound. If fetoscopy is used, the fetus tries to shield its eyes from the light with its hands.
Though, a baby born at this stage has no chances of surviving as lungs are too immature and the brain cannot control breathing and body temperature yet.
Third Trimester:
Month 7:A baby born at this stage has little chance of survival. If a baby is born betweek 7th and 8th months, it will have trouble with breathing and external oxygen supplies are required because tiny air sacs in the lungs are not yet ready. Cerebral cortex enlarges rapidly. The fetus stays awake for much time as neurological development advances. By the end of the seventh month, the fetus stays awake for 11 percent of its time. And by this time, the fetus can first feel pain.
Month 8:The fetus responds greater to external stimulation. Fetus can react to nearby sounds by movements. A layer of fat is added under the skin to help with temperature regulation. Also the fetus gets antibodies from mother's blood for immunization because the infant's own immune system will not start working until several months after birth.
Month 9:During the last weeks, the fetus prefers the tone and rhythm of the mother's voice. Just before birth, the fetus' waking time is almost 16 percent of its time. Since the size of the fetus has grown, it fills the uterus and has very less space to move. Physical activity is reduced also because of brain development. Normally the fetus stays in an upside-down position, partly because of gravity, as the head is heavier than rest of the body and partly because of the shape of the uterus. The fetus is ready to be born!
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