What happens in the embryo during the trimesters of pregnancy?

(January 14, 2010)


First trimester

The embryo is formed after the conception or fertilization of the egg during the 3rd week of a pregnancy in the first trimester. The embryo starts to develop rapidly after cellular division where the innermost cells form the fetus and the outer cells form the amniotic sac, umblical cord and the placenta. After the embryo has attached to the endometrial lining of the uterus the main external features of the embryo start to form and it grows rapidly. The embryo is oval in shape at this stage and changes to an elongated curve soon, with the head and tail becoming more distinguished. All the major organs begin to form starting with the vertebral column or spine and the brain. The heart begins to form and will start beating independently during the 8th week. The formation of tiny eyes can also be seen along with the arms and legs beginning to show as buds.

During the 4th week all the organs of the body have started to develop. The facial feaures begin to show as tiny bulges. The embryo is called a fetus after the 10th week of the pregnancy. The head is not in proportion and very large in size due to the developing brain. Also the liver growing in the abdomen makes it look large too. Arms and legs are more prominent after the 8th week and fingers begin to form. The basics of the thyroid, adrenal and pitutary glands which are the major hormone producing glands become present. The nails start to grow, the bones start to calcify and the tooth buds begin to show as well. The fetus starts to practice breathing and swallows the amniotic fluid. By the end of the first trimester the fetus has grown to about 3 inches in length.

Second trimester

All the organs are present by the end of the first trimester and the second is mainly devoted to the maturation of these organs. The fetus starts moving or rolling in the amniotic fluid in this trimester and the mother can feel these movements gradually. Hair growth occurs all over the body of the fetus at this stage, most of which disappears by the time it is born. The fetus grows rapidly and weighs close to a pound by the end of this trimester.  


Third trimester

At the beginning of this stage, the fetus is already fully developed. Thus the last 12 weeks are devoted to growth, with the fetus gaining over 28 grams in weight every day.  

Submitted by P T on January 14, 2010 at 08:29

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