A sprem fertilizes an egg and this creates a blastocyst.
After uterine implantation, the blastocyst develops into what is called an embryo. The embryo is receiving nourishment from the protective lining that initially contained the egg. This lining is called the corpus luteum. The embryo also receives nourishment from the mother's blood, which travels through the placenta. The placenta, at this point, is at a very early stage in its development. As the placenta develops, it will begin to provide more and more nourishment to the embryo in the form of hormones that are vital for the embryo's growth. The switch from the corpus luteum to the placenta as being the primary source of nourishment occurs between the eight and ten week point.
During week three, the embryo is extremely small. Its cells divide into three layers. The outer stratification is called the ectoderm. It consists of a flat layer of cells that form a tube that will ultimately become the spinal cord. The uppermost part of this tube will bulge out and form the primitive structure of the brain. Besides the nervous system, this top layer also contains the hair and skin of the embryo. The middle layer is called the mesoderm. It holds the early forms of the heart, the bones, cartilage, muscles, blood vessels, circulatory system, kidneys and reproductive organs. The heart and the circulatory system develop rapidly, the latter being the first organ to fully function in the embryo. The inner cell layer is called the endoderm. It is a tube in which the intestines, liver, pancreas and bladder will develop. As these organs develop, they form systems in which the organs function as a team. In this sense, the endoderm is responsible for developing the respiratory system, the digestive system, and the urinary tract.
In the fourth week, the embryo is about a tenth of an inch in size. It begins a period of rapid development. Although it is almost impossible to detect, the embryo has a heartbeat. The umbilical cord (what supplies the embryo with oxygen) is developing. There are dark spots where the eyes and ears will form and an opening appears that will eventually be the mouth. The digestive system forms. Also, tiny knob-like shapes appear, indicating the formation of the arms and legs.
Week 5. This is a common time when women realize that they are pregnant. This happens because the woman has gone an entire 4 weeks without having a period. The embryo is about a quarter of an inch long. The heart beats at a regular rhythm and pumps blood. It is about the size of a small pinhead and is already dividing into chambers. As the arms and legs sprout and the facial features develop, the embryo begins to take the shape of a small human. It still does not have fingers or toes. Beneath the mouth opening, tiny folds develop that will ultimately become the neck and jaw. The neural tube closes and the brain rapidly forms. Also, other major organs and body parts (eyes, bronchi, intestines, pancreas, and nostrils) appear.
Week 6
This is an exciting week because it is often when the embryo moves for the first time. It is doubtful that the moves are strong enough for the mother to discern them. The appendages are growing quickly, with elbows and knees becoming more and more observable. The hands and feet appear as formless masses with tiny marks where the toes and fingers will develop. With the rapid growth of the brain during week 5, the head is incredibly large compared to the rest of the body. The heart rate of the embryo is 160 beats per minute, almost twice as fast as the heart rate of the average adult. Other developments during week 6 are the formation of the gums and buds that will become teeth, and the strengthening of muscle fibers. The intestines also bond with the umbilical cord, allowing more and more nourishment to flow from the mother to the embryo.
In week seven the embryo is about three quarters of an inch long. The embryo moves much more often, although these motions can still be difficult to detect. The fingers and toes, still webbed-like in appearance, become much more apparent. The thumbs in particular develop during week seven. The body of the embryo has grown enough to where it has room for the intestines which leave the umbilical cord and enter the embryonic body. Other major developments include the tongue and palate distinctly forming, and the production in the liver of large quantities of red blood cells. Also, the cartilage becomes visible in the embryo. Week seven is a time of great development as the organs and body become more and more structured.
During week 8, the embryo becomes what is called a fetus. The baby is moving much more regularly and its movements can be detected by the mother. The fetus is about an inch long. The webbing around the toes and fingers has receded and joints are functioning. This means that the embryo can bend its elbows, wrists, and knees. At this time, most of the fetus' body parts (organs, nervous system) have begun to work.
With the beginning of the third month of the first trimester, a new phase begins. Most of the organs, muscle networks, and nerves have developed, making the remainder of the baby's stay inside the womb a time of growth. The fetus will now begin to enlarge. The fetus can be anywhere from an inch to two inches and weighs close to a third of an ounce. At week nine, the eyelids of the fetus bond so that the eye can develop. The intricacies of the inner ear are well-formed. There is a constant flow of blood between the fetus and the uterus, by which the fetus receives nourishment. The placenta now fully controls hormone levels and the baby's nourishment, taking over the role of the corpus luteum.
Week 10
The fetus can be about 2-3 inches during week ten. It weighs about a half ounce. The movements of the fetus become even more noticeable. The organ structure is very complex at this point. The kidneys are already processing waste and producing urine. The networks of muscles that line the intestines are contracting in order to allow food digestion. With the spinal cord in place, nerves are spreading to various parts of the body to establish the nervous system. With the fingers and toes nearly complete, the nails begin to develop. In some places, hair might grow. It is often during week ten that the genitals begin to show.
Week 11
During week eleven, the fetus will still be between 2-3 inches. The fetus is moving a lot. As a result of the movements, the fetus' muscles are growing stronger. Certain organs are completely functioning at this point. The liver is making bile and the pancreas is making insulin. Other major developments include the vocal cords and the basic structure of the face.
Week 12
By the last week of the first trimester, the fetus may be about 3 and one-half inches long. It may weight between 1 and 2 ounces. The face continues to form as the fetus becomes more and more human-like in appearance. As the eyes move in closer to the center of the face, the ears take position on the sides of the head. The neck takes shape, although it is still not strong enough to hold up the head. There is much more movement in the smaller and more specialized parts of the body, particularly the hands and feet. The networks of nerve cells are becoming more and more complex. This results in the development of reflexes.
Would you like the second and third trimester too?
2006-11-06 09:09:17
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answer #2
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answered by simplegrl 2
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