Early Pregnancy

• The first trimester of pregnancy is the first 12 weeks of pregnancy. The developing baby undergoes the most change in this period. Within days of fertilization, the egg implants and the embryo--a baby in its earliest stages of development--begins growing rapidly. Tissues form that will support and help the embryo to develop, such as the yolk sac, amniotic sac, and those cells that will eventually become the placenta and umbilical cord. Buds that will become the baby's limbs form in this stage and the heart starts to pump about 25 days after fertilization. After the second month of pregnancy, the baby is called a fetus.
  
  

12 Weeks

• By the end of the first trimester, the baby's spinal cord, bone cells, blood vessels and major organs have formed. Even the sex organs have formed at this point. It may move its limbs. Though very small, the baby is fully formed by the end of the third month of pregnancy. It is now about four inches in length, with a weight of about one ounce. During the first trimester, the pregnant woman often experiences nausea, fatigue and other uncomfortable symptoms as her body undergoes the changes necessary to support her baby.
  
  

Second Trimester

• In the second trimester, weeks 13 through 27, the baby begins to look more human. The skin forms, though it is practically transparent at first. The eyebrows and scalp may form as well. The bones continue to develop and the ears are almost in the place they will remain. During this stage, the baby begins to be sensitive to light and becomes capable of making facial expressions. The baby begins to develop stores of fat and the ability to hear. Vernix, a protective white covering, coats the baby's skin and fine hair covers his body. Many of the baby's organs are functioning, but continuing to mature. His eyes are closed. By the end of the second trimester, the baby may weigh up to two pounds, and he may be up to three times the length he was at the end of the first trimester. During this stage, the mother may feel more comfortable than she did in the first trimester of pregnancy.
  
  

Third Trimester

• In the third trimester--28 weeks through birth--the baby continues to develop and get ready for birth. Her eyes open and her pupils react to light. The soft hair covering her body drops off, and the womb becomes more cramped as the baby grows longer and heavier. Movements become more forceful at this age. The vernix gets thicker, and the baby develops the ability to suck.
  
  

Birth Preparations

• Towards the end of the third trimester, the baby may move into a head-down position to get ready for birth. The placenta delivers antibodies to the baby, which are important in protecting the baby in its first six months outside of the womb. The baby packs on more weight, and her organs continue to develop and mature in preparation for life outside of her mother. The mother may feel discomfort as a result of the growing weight of the baby, the uterus pushing against her organs and hormonal changes. Fatigue may return as well. However, many women also feel a sense of elation as their due dates grow closer.