During pregnancy, an embryo starts from a single cell and grows into a baby. Considerable amounts of growth and development occur in this time. The major body structures are formed during the first eight weeks of pregnancy (the embryonic period). Thereafter, the body organs, limbs and other systems continue to develop and grow.
First Trimester
During the first trimester, the embryo develops a human body structure---arms, legs, head, skeleton and internal organs---all from a single fertilized cell. An enormous amount of growth occurs, though the baby is less than two and a half inches long by the end of the embryonic period. The embryo has taken on the shape of a human and will continue to grow.
Second Trimester
During the second trimester the baby continues to get bigger. The finer features, such as fingernails and hair are formed. The organ systems of the body continue to develop and begin to become fully functional. The face is fully developed about halfway though the second trimester. Ultrasound evaluation performed at around 18 to 20 weeks is able to look at the baby and detect birth defects.
Third Trimester
The growing fetus now puts the majority of energy into getting bigger. The fetus becomes longer and rapidly gains fat. At about 31 weeks, the bones are fully developed and will continue to harden. The lungs are nearly fully developed though they keep gaining alveoli, the sacs of the lungs that exchange oxygen, until well after the baby is born. By about 36 weeks the baby would have a high chance for survival outside the womb, though medical interventions are likely needed.
Heart Growth and Development
The heart starts to develop as early as 24 days of gestation and the major heart structures are formed by the tenth week. The heart starts out as a single tube. That folds and creates dividers to make the four chambers of the heart. By the second trimester, many congenital heart defects can be seen during an ultrasound performed by a skilled ultrasound technologist. A hole in the heart, called the foramen ovale, remains open until birth or shortly thereafter. The baby in the womb does not need blood going to the lungs, since breathing has not begun.
Brain Growth and Development
The cells that will eventually become the brain are one of the first defining features of the developing embryo. Active development of the brain continues until birth. First, the major regions of the brain develop, and by about 4 months gestation something that generally looks like the brain exists. Afterwards, the lobes of the brain differentiate themselves. Gyri, the deep grooves of the brain, continue to form until birth. The brain continues to develop and make neuronal connections throughout life--most of the brain's growth takes place after birth.
References
- "Human Embryology"; William J. Larsen; 2001
- "Smith's Recognizable Patterns of Human Malformation"; Kenneth L. Jones; 2006
