The human fetus develops according to a set of precisely ordered and timed stages. The mother's egg is fertilized by the father's sperm and becomes a zygote---a single cell containing a new, complete set of DNA. This zygote rapidly divides and replicates itself, forming a ball of cells called the blastocyst over the next five days. Over the following weeks, it becomes an embryo, and then a fetus.
Week 8 of Fetus Development
At the start of the fetal stage, the fetus has already formed rudimentary versions of all of its basic body parts and organs. The heart is present and beating, making blood circulate through the developing arteries and veins. The spinal cord and the brain have also formed and are active, producing recordable electrical signals. As the muscles form, this activity will cause the fetus to move from time to time.
The arms and legs, with corresponding fingers and toes, are also present in week eight. The remainder of the pregnancy involves refining these existing body parts, increasing their size, complexity and coordination. The fetus floats within a protective, fluid-filled membrane called the amniotic sac. An umbilical cord provides a connection between the fetus and the placenta, an organ that provides a connection to the mother's blood stream and helps to regulate development.
Weeks 9-12 of Fetus Development
In this next stage of fetus development, the eyes of the fetus close and will remain closed for fully 19 weeks. The eyelids provide a protective covering for the eyes during this important period of their development. One of the most notable events of this stage is the differentiation of the genitals. If an ultrasound scan of the fetus is performed any time after week 12, it will often be possible to determine if the fetus is male or female.
Weeks 13-19 of Fetus Development
Starting in week 13 of the pregnancy, the fetus begins to grow a very fine hair called lanugo. Over the subsequent several weeks, the lanugo grows to cover the entire body of the fetus. Near the end of pregnancy, the lanugo is usually shed, but some of it is often still visible at the time of birth, especially if the birth is premature. According to the National Institutes of Health, the fetus begins to respond to sound in week 17. That is, if a loud noise is made near the mother's body, the fetus will often abruptly alter his or her level of activity. This suggests that, after 17 weeks, the fetus can hear. In the nineteenth week of the pregnancy, the fetus can first be seen making sucking motions with the mouth and tongue, pulling amniotic fluid into the mouth in the process.
Weeks 20-23 of Fetus Development
According to the Merck Online Medical Manual, week 20 is the time when the fetus grows large enough to fill the amniotic sac. The fetus has already been moving for at least 12 weeks at this point, but this will be the time when the expectant mother will first be able to feel those movements. The kicking and pushing sensations increase in frequency and strength for the rest of the pregnancy. They are especially noticeable for several minutes after the mother consumes something sweet such as fruit juice. After week 20, the heartbeat of the fetus can be heard using a stethoscope. It should be noted, however, that this doesn't always work, since the fetus shifts the position of the heart whenever he or she moves. Also the heartbeat of the mother is always much louder.
Week 24 of Fetus Development
Week 24 marks an important milestone in fetal development, because, after this week, the fetus will have some chance of survival outside the womb. According to the Merck Online Medical Manual, the lungs of a fetus develop the ability to perform gas exchange during this week of development. That is, the lungs will now be able to transfer oxygen from the air to the bloodstream and carbon dioxide from the bloodstream to the air. Normally the lungs won't be called upon to perform this function for about 13 more weeks, when the pregnancy will result in a full-term birth. However, if the fetus is born prematurely, this lung function will be essential for survival.
Weeks 25-32 of Fetus Development
The lungs of the fetus increase in strength and coordination during this stage, as the fetus makes breathing-like movements, pulling amniotic fluid into the lungs and then expelling it. An ultrasound scan may also show the fetus engaging in a new coordinated set of actions---putting the thumb into the mouth and sucking it. The eyelids of the fetus finally open again in week 28. For the remainder of the pregnancy, the fetus will blink the eyes open and closed from time to time. The fetus will also close the eyes during periods of sleep.
Weeks 33-42 of Fetus Development
A normal pregnancy ends with the birth of the fetus between weeks 38 and 42. During this final stage of development, the body of the fetus changes somewhat. For instance, the fingernails and toenails grow to the ends of the fingers and toes and somewhat beyond. The lanugo is also shed during this stage. Except for this, however, the most salient physical changes simply involve the fetus growing larger and stronger. While there are no significant anatomical changes, the brain of the fetus continues to develop rapidly. From the moment that the fetus is born, according to the What Babies Can Do DVD, he or she will be able to see, hear, and make some sense of the surrounding environment.
