A human fetus develops over the course of about 38 weeks of pregnancy. This developmental process follows a precisely ordered and timed set of stages. Scientists have characterized these stages in enough detail to fill several books. Several stages in particular are the most salient milestones of fetus development.
Development Before the Fetal Stage
The mother's egg is fertilized by the father's sperm in one of her fallopian tubes, to produce a single cell called a zygote. This zygote travels down the fallopian tube and into the uterus, where it becomes embedded in the uterine wall. As it does so, the zygote rapidly divides and replicates itself, producing a ball of cells called the blastocyst. Part of the blastocyst develops into the placenta, an organ that serves as a connection to the mother and helps regulate development. The other part of the blastocyst develops into an embryo and, by week eight of the pregnancy, into a fetus.
Eighth Week
The fetal development stage officially begins during the eighth week of pregnancy. The fetus floats within a fluid-filled membrane call the amniotic sac, attached to the placenta by the umbilical cord. All of the oxygen and nutrients needed to support the development of the fetus come from the mother's blood stream, through the placenta, via the umbilical cord. The embryonic stage that precedes this is characterized by the emergence of the foundations of all of the body parts and organs. During the fetal stage, these rudimentary parts grow in size and complexity, becoming all of the functioning parts needed for life outside the womb. By week eight, the heart is present and beating, causing blood to circulate through the body. The spinal cord and the brain are both formed as well and produce recordable electrical activity. As the muscles develop, this activity will cause the fetus to move. The arms and legs have sprouted by week eight as well, with fingers and toes on their ends.
Ninth through 12th Weeks
During the ninth week of the pregnancy, the eyelids of the fetus close, providing a protective cover as the eyes develop over the following 18 weeks. Also near the start of this stage, the genitals of the fetus differentiate, according to the National Institutes of Health. An ultrasound scan can show whether the fetus is male or female beyond this stage. While the main body parts are all present, their relative size is different from those of an adult or even a newborn baby. For instance, the head of the fetus makes up half of his overall height.
13th through 19th Weeks
During the 13th week, the fetus sprouts a very fine hair called lanugo. It initially appears on the head but eventually covers the entire body. Typically, this lanugo is shed before the end of the pregnancy but some is occasionally visible at birth, especially if the birth is premature. Several studies have shown that the fetus responds to sound by week 17, according to the National Institutes of Health. If a loud noise is made outside the womb, the fetus will exhibit an abrupt change in his level of activity. This result suggests that 17-week-old fetuses can hear. As this stage progresses, the movements of the fetus become progressively more coordinated. For instance, by week 19, the fetus is capable of making sucking motions with the mouth and tongue.
20th through 23rd Weeks
According to the Merck Online Medical Manual, the fetus grows large enough to fill the amniotic sac at around week 20. The fetus has been moving for many weeks before this stage, but now that he is large enough, the expectant mother will begin to feel kicking and pushing movements. These movements will be felt with increasing frequency as the pregnancy progresses. The heartbeat of the developing fetus can first be heard with a stethoscope at this stage.
24th Week
According to the Merck Online Medical Manual, week 24 marks an important milestone in the development of the fetus, when the lungs develop the ability to perform gas exchange--deriving oxygen from and expelling carbon dioxide to air. Under normal circumstances, the fetus's lungs do not do this, since the umbilical cord and the placenta provide a connection to the mother's bloodstream. If the fetus is born prematurely beyond this stage, however, he or she will have some chance of survival.
25th through 32nd Weeks
During week 28, the eyelids, which have been closed since week nine, open again and begin to periodically blink. The lungs of the fetus begin to expand and contract in breathing-like motions, pulling in and expelling amniotic fluid as they do so. Even more coordinated movements also appear. For instance, an ultrasound scan will often show the infant putting his or her thumb into the mouth and sucking on it.
33rd through 40th Weeks
This stage is when the lanugo is typically shed, and the nails grow to reach the ends of the fingers and toes. In the final weeks of the pregnancy, the fetus continues to grow in size and strength, but there are no major, qualitative physical changes. The brain continues to develop rapidly, however. According to the "What Babies Can Do" DVD, when the baby is born, he or she will already be able to see, hear, and make some sense of the surrounding world.
