Though their time in the uterus is relatively short, babies undergo amazing changes from conception to birth. After only about 40 weeks, they have changed from a tiny ball of cells to a full-term infant, ready to survive in the world. During each stage of pregnancy, these babies experience specific developmental milestones.
Three to 10 Weeks
Weeks three to 10 are considered the embryo stage of the baby's development in pregnancy. During this stage, the baby changes from a ball of cells into a more human shape. By the end of the embryo stage, the baby is about an inch long, with buds for arms and legs. It also has eyes, ears and internal organs. Perhaps the most exciting milestone during the embryo stage is the visible heartbeat on an ultrasound, which the American Pregnancy Association reports can happen as early as week six.
10 to 13 Weeks
At the 10th week of pregnancy, the Merck Manuals Online Medical Library (Merck) reports that the embryo is considered a fetus. During the rest of the first trimester (through week 13) the baby develops eyelids and teeth. Other significant events during the first trimester include the development of the external sex organs and the baby's ability to open and close his hands. By week 13, the baby is about 3 inches in length.
14 to 20 Weeks
Week 14 marks the beginning of the second trimester, which lasts until week 28. During this stage, the baby's skin becomes transparent. By week 20, the skin is covered with tiny hairs called lanugo, and a protective coating called vernix. The baby also develops eyelashes and fingernails, and may practice swallowing amniotic fluid. Week 20 marks the halfway point of a pregnancy. By this time, an ultrasound can reveal the baby's sex.
21 to 28 Weeks
During the end of the second trimester, the baby develops distinct sleep and wake cycles, which his mother may be aware of. The sexual organs continue to grow and develop, and by now the baby has his own set of fingerprints and footprints. One of the milestones of the end of the second trimester is the baby's ability to survive outside the womb if born prematurely. According to Merck, the first time this is possible is week 24. By the end of the second trimester, the baby weighs a few pounds, and is over a foot long.
29 to 36 Weeks
Week 29 starts the third trimester. During this time, the baby can open and close its eyes and may even move in response to light. He may also take practice breaths. The lanugo falls off during this time, and vernix continues to build. By week 36, the baby is about 4 lbs., and is nearly as long as he will be at birth. A baby born up until week 36 is considered pre-term.
Weeks 37 and On
At week 37, the baby's organs and lungs are mature enough to function on their own, and the baby is considered to be full-term. He will continue to gain weight until his birth day. One of the last changes to take place during this stage is the assumption of the head-down position, in preparation for birth. By this time, he is full-sized, and should weigh between 6 and 10 lbs.
