Between the early months following conception and childbirth, dramatic changes take place in the developing human fetus. From the initial embryonic stage in which the fetus is entirely dependent on the mother for survival, a fetus develops into an incredibly complex organism capable of surviving on its own. Fetal development can be divided into distinct stages.
Gestational Stage
The single cell resulting from the combination of a mother's egg cell and a single sperm is called a zygote. This cell divides into a cluster of cells, called a blastocyst, which contains an inner group of cells which will become the embryo and an outer shell will become membranes that protect and nourish the developing fetus.
Embryonic Stage
During the embryonic period, which lasts until about the eighth week of pregnancy, the brain, spinal cord, heart and gastrointestinal tract begin to develop. At weeks four and five, arm and leg buds are visible and eyes and ear structures have begun to form. By week seven, all of the embryo's essential organs have begun to form.
Fetal Stage
Between weeks eight and 12, the beginning of the fetal period, the developing fetus's eyelids close and will remain closed until week 28. At this stage, the face and genitals are well formed, the fetus can make a fist with its fingers, and tooth buds for baby teeth appear. The head accounts for nearly half of the size of the fetus. Fine hair on the head develops between weeks 13 and 16. By week 20, this hair covers the entire body, nails appear on the toes and fingers, and the baby's heartbeat can be heard with a stethoscope. At week 24, all eye components are formed and air sacs develop in the lungs. Rapid brain development occurs between weeks 25 and 28, and the nervous system is developed enough at this stage to control a number of body functions. Body fat increases between weeks 29 and 36. In the final weeks of development, small breast buds are seen in both males and females, and head hair is coarse and thick.
How to Take Care of Your Developing Baby
Proper prenatal care and a healthy diet are important for pregnant women. Alcohol, some medications, recreational drugs, nutritional deficiencies, X-rays or radiation therapy and infection can all interfere with a baby's development, especially in the early stages of pregnancy when rapid growth is taking place.
Fetal Development in Multiple Births
Fetuses in multiple births develop at the same rate as single fetuses for the first two trimesters, and then their growth rate decreases from about week 26 on. Multiple birth children may have early developmental delays, but usually catch up with single birth children over time.
