From conception to birth, a baby undergoes dramatic changes in its mother's uterus. Fertilization occurs at the two week mark, and the baby is only a small cluster of cells. By the 40 week mark, however, that cluster has become a full-grown infant. Each stage of growth and development in the uterus is characterized by specific milestones.
Blastocyst
A few days after fertilization (around the second week of pregnancy), the hollow cluster of cells that will become a baby is called a blastocyst. The blastocyst stage begins as the fertilized egg divides rapidly, becoming a hollow ball of cells. It travels down the fallopian tube and attaches to the uterus wall. According to the Merck Manuals Online Medical Library, the placenta and amniotic sac begin to develop during the blastocyst stage.
Embryo
Around the three week mark, the ball of cells becomes an embryo. According to the US Department of Health and Human Services (HHS), the baby is approximately 1/25 of an inch at this stage. However, it is developing rapidly from a tiny ball to something resembling a human. A head begins to take shape, and tiny buds form, which will become arms and legs. During this stage, the brain and spinal cord also begin to develop. According to Merck, by the 10th week of pregnancy, most of the major organs have already formed. By the end of the embryo stage, the HHS reports that the embryo is around an inch long.
Fetus
From week 10 until the end of pregnancy, the baby (which now looks like a baby) is called a fetus. During the rest of the first trimester (until week 13), the baby's external sex organs become visible; however, the American Pregnancy Association reports that they are still too small to be seen on an ultrasound. The baby now fills up the uterus, and both will continue to grow together for the duration of the pregnancy. By the end of the first trimester, the baby has eyelids (which remain closed for some time), can make a fist and is approximately 3 inches long.
Second Trimester
During the second trimester (weeks 14 to 28), the baby has transparent skin covered in fine hair (languno) and a waxy protective coating (vernix). He develops fingernails and eyelashes, and has distinguishable fingerprints and footprints. Hair forms on his head. During this stage, the baby develops his startle reflex, and his movements become large enough to be detected by his mother. According to Merck, by 24 weeks, the baby has a chance for survival outside of the uterus if born prematurely. By the end of the second trimester, the baby is approximately a foot long.
Third Trimester
In the third trimester (weeks 29 and on), the baby practices for life outside the womb. He opens and closes his eyes, and his movements become larger and stronger. The bones and lungs form and mature, and body fat develops. The lanugo begins to fall off, and is usually completely gone by the time the baby is born. Around week 33 or so, most babies will begin to turn, assuming the head-down position in preparation for birth. By the end of the third trimester, the American Pregnancy Association reports that the baby is usually between 19 to 21 inches long, and weighs around 6 to 10 lbs.
