Before the fetus is born, a precisely ordered and timed set of developments takes place. Over the course of eight weeks, a single, fertilized egg cell grows into a ball of cells called the blastocyst, then an embryo and then a fetus. The fetus will develop to full term, ready for life outside the womb, during the subsequent 29 to 34 weeks of pregnancy.
Week 8
By the start of this stage of pregnancy, the developing fetus possesses all of the primary body parts and organs. The arms are present, with fingers growing at their ends. The legs and toes are also already present. The heart beats within the chest of the fetus, causing blood to circulate through the developing arteries and veins. The brain and spinal cord are present and developing rapidly. They already produce recordable electrical activity that stimulates the growing muscles of the fetus to contract from time to time, causing the fetus to move. The fetus is attached, via an umbilical cord, to the placenta, an organ that provides a connection to the mother and helps to regulate development. As the fetus floats within a fluid-filled membrane called the amniotic sac, this umbilical cord provides all of the oxygen and nutrients needed to support development.
9 to 12 Weeks
During this stage of development, the eyes of the fetus close and remain closed for approximately 19 weeks. During this extended period, the eyelids provide a protective cover as the eyes develop. Near the end of this stage, the genitals of the fetus differentiate. If an ultrasound scan is performed from this point on, it will usually be possible to determine whether the fetus is male or female.
13 to 19 Weeks
Starting in week 13, the fetus begins to grow a very fine hair called lanugo. Eventually, this hair will cover the entire body. The lanugo is usually shed prior to birth, but it is not unusual for some to remain, especially for fetuses born prematurely. In week 17, according to the National Institutes of Health, the fetus begins to respond to sound. If a loud noise is made near the mother, the fetus will often abruptly change his level of activity. Findings such as this suggest that 17-week-old fetuses can hear. Over the course of the pregnancy, the movements of the fetus continually become more coordinated and complex. For example, near the end of this phase of development, the fetus will begin to make sucking motions with the mouth and tongue. As he does, amniotic fluid is pulled into the mouth and expelled.
20 to 23 Weeks
According to the Merck Online Medical Manual, the fetus grows to fill the amniotic sac during week 20 of the pregnancy. At this same time, the expectant mother will begin feeling kicking and pushing movements of the fetus. This will be especially noticeable during periods of time after the mother consumes something sweet, such as fruit or juice. During this stage, the heartbeats of the fetus can first be heard using a stethoscope. It should be noted that this will not always be possible, however, both because the heartbeat of the mother is much louder and because the position of the fetus's heart changes whenever the fetus moves.
24 Weeks
During week 24 of the pregnancy, according to the Merck Online Medical Manual, the lungs of the fetus develop the ability to transfer oxygen from the air into the bloodstream and to transfer carbon dioxide from the blood to the air. The lungs of the fetus do not normally perform this gas exchange process at this stage. The umbilical cord delivers all of the necessary oxygen, and the fetus is fully submerged in amniotic fluid, which fills the lungs. If, however, the fetus is born prematurely, the onset of this lung function will provide the fetus with some chance of survival.
25 to 32 Weeks
In this stage, the eyes of the fetus open again. After they do, they periodically blink open and closed, just as they will after the fetus is born. When the fetus sleeps, the eyes remain closed for several hours at a time. The fetus begins to make breathing-like motions, pulling amniotic fluid into the lungs and expelling it as they do. Ultrasound scans performed near the end of this phase will sometimes reveal the new ability of the fetus to put the thumb into the mouth and suck on it.
33 to 42 Weeks
In this final stage of development before the fetus is born, the body grows larger and stronger, but there are few major changes. The lanugo is shed, and the nails grow to the ends of the fingers and toes, but there are no other milestones of physical growth. The brain of the fetus does continue to develop at a rapid pace, however. According to the What Babies Can Do DVD, when the fetus is born, he will immediately be able to see, hear and make some sense of his surroundings. For instance, the fetus will already possess the ability to follow objects with his eyes and will exhibit a preference for looking at faces. This is impressive given the fact that the fetus has grown entirely in the dark over the preceding nine months.
