A baby goes through tremendous growth and development during its 40 weeks in a mother's uterus. With so many changes occurring, prenatal care is essential to monitor the health of the baby and the mother to ensure a healthy pregnancy and delivery.
First Trimester
The first trimester is a critical time in pregnancy when most fetal development occurs. The baby's heart begins to beat, the brain, spinal cord and organs begin to develop. According to MayoClinic.com, the baby begins to move its arms and legs around the eighth week. Although the sex of the baby is determined at conception, the genitals form around the eleventh week of pregnancy. The baby begins the first trimester as a single cell and by the end of the 12 weeks reaches about 2 1/2 inches and weighs 1/2 oz.
Second Trimester
MayoClinic.com defines the second trimester as beginning with week 13 and ending with week 27. During this time the skeleton is forming, hair patterns are developing and hair follicles begin to grow. The baby begins to store fat so as to stay warm after birth. Baby's hearing develops, and at around 20 weeks, parents can learn its sex. According to the American Pregnancy Association, fine hair called lanugo begins to form. Lanugo protects the baby's skin from the amniotic fluid. Fingerprints and footprints have formed, and an expectant mom may feel her baby's first kicks. At the end of the second trimester, the baby is around 9 inches long and weighs about 2 lbs.
Third Trimester
The third and final trimester ends when the baby is born, around 40 weeks. During the third trimester the baby's eyes open and the pupils can dilate. Around the 29th week of pregnancy the bones are fully formed and the baby can grip with its hands. According to the American Pregnancy Association, at around 33 weeks of pregnancy the baby begins to turn its body so that its head is positioned toward the birth canal. The baby is considered full term at 38 weeks and is ready for birth. At this time the baby may be around 19 to 21 inches long and weigh about 7 1/2 lbs.
