A normal pregnancy is 40 weeks in duration and totals 9 months of gestation. This is commonly broken down into trimesters lasting from 12 to 14 weeks each. There are many exciting milestones during fetal development and the process first begins immediately upon fertilization. Pregnancy.org indicates that this is when gender is determined. Two weeks after conception the neural tube forms, which is a significant part of the nervous system. The neural tube is the foundation for the brain, spinal cord, hair and skin. The American Pregnancy Association indicates that in early pregnancy weeks the heart and digestive systems begin to form.
Trimester One
Around the fifth week of pregnancy the embryo begins development and the layers of skin form. The Mayo Clinic indicates there are three layers of the embryo: ectoderm, mesoderm, and endoderm. The ectoderm is the outer layer of skin. During this time the nervous systems also begin to develop as well as the eyes and inner parts of the ears. The mesoderm is the middle layer of skin and helps to protect the bones and muscles of the baby. The endoderm is a a mucous lined tube that aids in the development of the baby's lungs, intestines, and bladder. According to the American Pregnancy Association, by the end of the first trimester the baby transitions from being an embryo to a fetus with clearly formed genitalia and a full facial profile.
Trimester Two
During the second trimester the fetus has functioning organs and muscles. Although gender is established in the first trimester, it becomes visible on a sonogram. The second trimester is a time of rapid growth for the fetus and some movement of the fetus is detectable by the mother. The Mayo Clinic indicates that the baby begins to pick up sounds such as heart beats and stomach rumbling. The startle response is often formed during this time. In addition to continued organ development the baby also develops finger prints, fine hair and fingernails.
Trimester Three
The third trimester signifies the final stages of development for the baby. The fetus is storing fat, gaining length, and beginning rhythmic breathing. As the weeks get closer to delivery the fetus begins to descend head down toward the birth canal. The American Pregnancy Association identifies weeks 37 to 40 as full term development. All of the organs are developed by the end of this trimester and the baby is the length and weight he will be for delivery.
