Week 14: The neck is well defined and the body is now covered in a very thin, soft layer of hair. The body is growing faster now and the liver has begun to secrete bile. Your baby can now move the muscles in its face and is even capable of some expressions.
Week 15: The baby is about 4 inches long. The legs are now longer than the arms. Some women (like me) feel movement this early. Although the eyelids are still closed, the baby can sense light through them. Taste buds on the tongue and eyebrows have also begun to form.
Week 16: The heart now pumps on average of 25 quarts of blood a day. Although the baby receives all of its oxygen via the umbilical cord, it has learned to breathe. Inhaling and exhaling will help its lungs to properly develop.
Week 17: This is the average time a woman will begin to feel the baby move. The skeleton beings to develop from cartilage to bone. On average, the baby is about 5 inches long and weighs about 5 ounces.
Week 18: If the baby is a girl, the uterus and Fallopian tubes are formed and in place. If it is a boy, his genitals are noticeable. Myelin, a protective covering over the nerves, and vernix, a protective coating over the skin both begin to form.
Week 19: Many women choose to have an ultrasound at this point in their pregnancy, although some do it earlier. This is a crucial time for sensory development. The baby's brain is designating specialized areas for smell, taste, hearing, vision, and touch.
Week 20: The halfway point! The baby is swallowing more, which is good for the digestive system. It also produces meconium, a black, sticky substance that is the result of cell loss, digestive secretion, and swallowed amniotic fluid. (After birth, it takes about two or three days for the baby to rid itself of the meconium and produce proper poop.) The baby can hear sounds outside of the uterus and may startle in reaction to loud sounds.
Week 21: The baby begins white blood cell production. If you are having a girl, her vagina has finished forming. The baby will now enter a growth spurt and will put on more weight from week to week.
Week 22: The baby weighs almost one pound. Until last year, it was believed that no baby born this early could survive. Amillia Sonja Taylor, the world's smallest newborn changed that in a big way. Born at a gestational age of only 21 weeks 6 days, she beat the odds and lived. She is now four months old. It is uncertain what sort of long term developmental delays she will have, but the mild brain hemorrhage and digestive troubles present at birth are not expected to cause any long term problems.
Week 23: At this stage, a prematurely born baby has about a 5%-15% chance of survival. The baby weighs just over a pound.
Week 24: The baby lungs are developing branches and cells that produce surfactant, a substance that helps the air sacs inflate easily. On average, the baby is about 1 1/4 lb and about a foot long.
Week 25: The baby has grown another inch and gained another quarter pound. Reflexes, such as swallowing and grasping are also improving.
Week 26: The baby continues to grow and is on average 2lbs. If the baby is a boy, the testicles are beginning to descend into the scrotum. This process takes on average two or three days to complete.
Week 27: Although the baby's lungs are immature, they are capable of functioning with the help of a steroid. The baby can also open and close its eyes.