Over the course of approximately 38 weeks, a single, fertilized egg develops into a full-term fetus, ready to be born as a new baby. This process of development progresses through a remarkably consistent set of stages. These stages have been characterized in tremendous detail---enough to fill several books. Here are a few of the biggest milestones of the development.
Growth That Leads Up to the Fetal Stage
At first, the fertilized egg is referred to as a zygote; much development needs to take place before it will be called a fetus. The zygote implants itself in the wall of the mother's uterus within several days after fertilization. It divides and replicates itself, growing into a ball of cells called the blastocyst. This ball of cells grows larger and divides into two parts; one part becomes the placenta, and the other part becomes the embryo. The placenta serves as a connection to the mother and helps to regulate fetal development. Over approximately 7 weeks, the embryo develops into the fetus.
Fetal Stage, Week 8
By the start of the fetal stage, the fetus itself floats within the amniotic sac---a thin, fluid-filled membrane formed by part of the placenta. The fetus obtains all of its oxygen and nutrients from the mother's bloodstream via the umbilical cord. By the start of the fetal stage, all of the main body parts are present. The heart, brain and spinal cord are all formed. The heart beats and pumps blood. The fetus has arms and legs with fingers and toes on them. The brain is active, producing electrical activity that stimulates the muscles to contract and causes the fetus to move.
Fetal Stage, Weeks 9-12
Around week 9, the fetus's eyelids close, covering and protecting the developing eyes. The eyes will stay closed until the 28th week of development. According to the National Institutes of Health, the genitals become differentiated near the start of this fetal stage, making it possible to determine if the fetus is male or female based on an ultrasound image. While the fetus has all of its main body parts, they are very different in relative proportion to those seen in an adult. For instance, the head makes up fully half of the fetus's overall height.
Fetal Stage, Weeks 13-19
During this stage of development, the fetus grows lanugo---a very fine hair that covers the entire body. This hair will usually be shed prior to birth, but some may remain for some newborns, especially for children born prematurely. According to the National Institutes of Health, the fetus begins to respond to sound during week 17. By this stage of development, it seems that the fetus can hear. Movements become more coordinated for the fetus during this stage as well. The fetus may even make sucking motions with its mouth, pulling amniotic fluid into the mouth in the process.
Fetal Stage, Weeks 20-23
According to the Merck Online Medical Manual, this is the stage during which the fetus grows to fill the amniotic sac. As it does, the fetal body movements---especially kicking---can begin to be felt by the expectant mother. This increases with frequency during the remainder of the stage. Also during this stage, the heartbeat of the fetus becomes strong enough to be heard using a stethoscope.
Fetal Stage, Week 24
During week 24, the lungs begin to develop air sacs that can function for gas exchange---transferring oxygen from the air into the blood and carbon dioxide from the blood into the air. During a normal pregnancy, the lungs won't be called upon to do this, of course, as the fetus gets all of the oxygen that it needs through the umbilical cord. According to the Merck Online Medical Manual, if the fetus is born beyond this stage, there is some chance of survival due to the onset of this lung function.
Fetal Stage, Weeks 25-32
The most dramatic developments that take place during this stage are in the nervous system and brain, which become increasingly active and functional. The fetus's eyes periodically blink open and close. The fetus begins to expand and contract the lungs in breathing-like movements, pulling amniotic fluid into the lungs and then expelling it. Even thumb sucking can be seen during this stage.
Fetal Stage, Weeks 33-40
The fetus grows larger during this final stage, but there are no dramatic changes in anatomy. The lanugo finishes shedding. The fingernails and toenails grow to the ends of the fingers and toes. At some point near the end of this phase, the fetus will be born.
