During a 38-week pregnancy, one fertilized egg cell develops into a fetus that is soon ready to become a newborn baby. The process of fetal development consists of a set of stages that unfold with consistent order and timing. Fetal development stages include some major milestones of what is a fascinating process.
Week 8
The fetal development stage begins around the eighth week of the pregnancy. At the beginning of this stage, the placenta has formed a thin, fluid-filled membrane called the amniotic sac. The fetus floats in the amniotic fluid contained within this protective sac, attached to the placenta and the mother via the umbilical cord. All of the oxygen and nutrients needed for development are delivered from the mother's bloodstream to the fetus via this cord. Even at the beginning of this stage, the fetus has rudimentary forms of all of its main body parts. The heart is present, beats, and pumps to circulate blood around the body of the fetus. The brain and spinal cord are both formed and active. The brain produces recordable electrical activity that causes the muscles to contract and the body to move. Arms and legs are both present as well, with corresponding fingers and toes.
Weeks 9 to 12
Several significant changes happen between weeks 9 and 12 of the pregnancy. The eyelids of the fetus close during week 9 and stay closed until week 28. The eyelids cover and protect the eyes during this crucial stage of development. According to the National Institutes of Health, the genitals become differentiated near the start of this fetal stage. If an ultrasound scan is performed beyond this age, it may be possible for a medical technician to determine if the fetus is male or female. Although all of the major body parts are already present a this stage, it is notable that their relative size is very different from those of an adult, or even a newborn baby. The head, for instance, is very large, making up about half of the overall height of the fetus.
Weeks 13 to 19
Around week 13, the fetus begins to grow a very fine hair called lanugo. It starts on the head, but eventually covers the entire body. Most of this hair will be shed prior to the time that the baby is born, but it is not unusual to see some on newborns, particularly if the child is born prematurely. According to the National Institutes of Health, the fetus begins to respond to sound during week 17. If a loud noise is made outside the womb, the fetus typically exhibits an abrupt change in activity level. This finding suggests that the fetus can hear. By week 19, the fetus will be making more coordinated movements than he or she made at week 13. By week 19, the fetus makes sucking motions with the mouth and tongue, pulling amniotic fluid into the mouth and expelling it.
Weeks 20-23
One of the most salient experiences that a mother will have during pregnancy is the first feeling of the fetus moving inside her. Around week 20, the fetus grows to fill the amniotic sac, according the Merck Online Medical Manual. The fetus moves a great deal prior to this, but it is at this stage of development that the expectant mother can first feel that movement. The kicking and pushing movements will be felt more and more frequently over the subsequent weeks. It is also during this stage that the fetus's heart beat can first be heard using a stethoscope, although this can be difficult since the heartbeat of the mother is still much louder.
Week 24
If the fetus is born beyond week 24, according to the Merck Online Medical Manual, it has some chance of survival outside the womb. This is because if the onset of the ability of the lungs to perform gas exchange---absorbing oxygen from air and expelling carbon dioxide. The lungs don't normally need to do this, of course, since the fetus obtains oxygen and eliminates carbon dioxide via the umbilical cord.
Weeks 25 to 32
The fetus will begin to expand and contract the lungs during this stage, pulling in and expelling amniotic fluid as he or she does so. The brain and nervous system also become increasingly active. The eyes open again and begin to periodically blink. More coordinated movements also begin to occur. For instance, the fetus will periodically put the thumb into the mouth and suck on it.
Weeks 33 to 40
There are no dramatic, qualitative developments during the final weeks of the pregnancy, although the fetus continues to grow larger and stronger by the day. Any remaining lanugo is usually shed. The fingernails and toenails continue to grow, usually extending beyond the ends of the fingers and toes. Near the end of this stage, the fetus will typically be born. According to the What Babies Can Do DVD, the newborn will already be capable of seeing, hearing, and making some sense of the surrounding world.
