The Three Trimesters of Fetal Development
The First Trimester
The first trimester of pregnancy is very exciting, and your baby experiences many changes. The first trimester of pregnancy begins at conception and ends at the twelfth week. During the first four weeks of pregnancy your baby is considered an embryo. It’s nervous system, heart, arm and leg buds begin to form. By the eighth week of pregnancy your baby is now considered a fetus. During this time, it has developed all of its major organs, they begin to develop facial features, their arms and legs grow longer, and fingers and toes begin to form. Your fetus’s umbilical cord is now visible, it has begun to develop sex organs, and has a beating heart. At the end of the first trimester, week twelve, your baby’s nerves and muscles begin to work, and your baby is able to make a fist. Your baby should now be approximately three inches long and weigh about an ounce. Your baby’s sex organs are now fully formed, and it is possible to tell if it is a boy or a girl.
The Second Trimester
The second trimester begins at week thirteen and ends at week twenty-eight. By week sixteen, your baby’s skin has begun to develop, however it is nearly translucent at this stage. Your baby has developed a sucking reflex, your baby is now approximately four to five inches long and weighs approximately three ounces. At twenty weeks, you are half way through your pregnancy. Your baby is becoming more active, it is possible to feel your baby’s movements and kicks at this stage. Your baby can now hear and swallow, their fingernails and toenails have developed, and your baby now has eyebrows and eyelashes. Although your half way through your pregnancy, your baby still has a lot of growing to do. At twenty weeks your baby should be about six inches long and weigh about nine ounces. At week twenty-four your baby has distinguished fingerprints and footprints, their taste buds have formed on their tongue, and hair begins to grow on your baby’s head. Your baby’s lungs are now formed; however, they do not work yet, and your baby now has a regular sleep schedule. Your baby’s sex organs become more fully developed, and your baby is now storing fat and should weigh about one and a half pounds and is approximately twelve inches long.
The Third Trimester
The third trimester begins at week twenty-nine and ends at birth. At week thirty-two, your baby’s bones are soft, but fully formed. Your baby begins to move more, they can open and close their eyes, and they practice breathing although the lungs are not fully developed. Your baby has begun gain weight at about half a pound a week, their body has begun to store vitamins. They should weigh between four and four and a half pounds and be approximately fifteen to seventeen inches long. At thirty-six weeks, your baby’s body fat increases and they have grown to the point where there is not much room to move around. You should still be able to feel your baby move; however, the kicks and movements are not as strong. Your baby should now weigh between six to six and a half pounds and should be about sixteen to nineteen inches long. At the end of week thirty-seven your baby is considered full term; their organs are now fully developed and are able to function on their own. Your baby should soon move into a head down position to prepare for birth. The average birth weight is between six and nine pounds and average length is between nineteen to twenty-one inches. However, a healthy baby can be many sizes and weights.
https://www.onhealth.com/content/1/pregnancy_stages_trimesters.