A normal pregnancy lasts 38 to 42 weeks, during which a single, fertilized egg grows into a full-term fetus, ready for life outside the womb. The growth that takes place before birth progresses through a set of precisely timed and ordered stages that have been characterized in detail by developmental scientists.
Growth Leading Up to the Fetal Stage
The father's sperm usually fertilizes the mother's egg within one of her fallopian tubes. Half of the father's and half of the mother's DNA combine to form a new, complete set of genes. The fertilized egg is called a zygote. It immediately begins to divide and replicate itself as it travels down the fallopian tube and into the uterus, where it embeds itself in the uterine wall. Within a few days, the zygote has developed into a ball of several hundred cells known as the blastocyst. By week eight of the pregnancy, the blastocyst has developed into an embryo and then a fetus, which possesses all of the major parts and organs of a human body.
Week Eight
At week eight, the fetus floats within a thin, fluid-filled membrane known as the amniotic sac, connected to the mother via an umbilical cord. The fetus receives all of the oxygen and nutrients needed for development from the mother's blood stream, by way of an organ called the placenta, and then via this umbilical cord. By week eight, the heart is present and beating, which causes blood to circulate through the fetus' body within its developing arteries and veins. The brain and the spinal cord are also present in the fetus at this stage, and they produce recordable electrical activity. As the muscles and nerves form, this activity causes the body of the fetus to move from time to time. The fetus also possesses arms and legs with fingers and toes at their ends.
Weeks Nine Through 12
In the ninth week of the pregnancy, the eyelids of the fetus close and remain so for approximately 18 weeks, providing a protective cover as the eyes develop. The genitals become differentiated in week nine, according to the National Institutes of Health. If an ultrasound scan is performed beyond this stage of the pregnancy, it is often possible to determine whether the fetus is male or female. It is notable that the body parts of the fetus, while present, are different in relative size than those of an adult or even a newborn. Perhaps the most salient of these size differences is that of the head, which makes up fully half of the fetus's overall length.
Weeks 13-19
At the beginning of this stage, the fetus starts to grow lanugo, a fine hair that eventually covers his or her entire body. The lanugo is usually shed before the end of the pregnancy, but some is occasionally present at the time of birth, especially if the birth is premature. In week 17, according to the National Institutes of Health, a fetus will often respond to a loud noise by abruptly changing his or her level of activity. This type of result suggests that a fetus is able to hear from this stage onward. A 19-week-old fetus starts to exhibit the first sucking movements with the mouth and tongue, pulling amniotic fluid into the mouth and expelling it.
Weeks 20-23
At approximately 20 weeks, an expectant mother will start to feel kicking and pushing movements by the fetus. The fetus has been moving for many weeks already at this point, but it can now be felt because, according to the Merck Online Medical Manual, this is the stage at which the fetus has grown to fill the amniotic sac. For the remainder of the pregnancy, the mother will feel these movements with increasing frequency, especially after consuming sweet foods and drinks such as fruit juice. This is also the earliest stage at which the heartbeat of the fetus can be heard with a stethoscope on the mother's belly. It should be noted that this will sometimes be a challenging task, however, since the position of the fetus heart changes as he or she moves, and the heartbeat of the mother is much louder.
Week 24
According to the Merck Online Medical Manual, the lungs of the fetus develop the ability to derive oxygen from air and to emit carbon dioxide into air at week 24. This is an important milestone, because this development will provide the fetus with some chance of survival if he or she is born prematurely beyond this stage. Under normal circumstances, however, the lungs do not perform gas exchange processes at this point. All oxygen absorption and carbon dioxide emission happens via the umbilical cord and the mother's bloodstream.
Weeks 25-32
The brain and nervous system of the fetus become increasingly active during these weeks, causing correspondingly more complex movements. The lungs, for instance, begin to expand and contract in breathing movements. As they do, the fetus inhales and exhales amniotic fluid. The eyelids open for the first time since week nine, after which the fetus's eyes periodically blink open and closed. If an ultrasound scan is performed at this stage, the fetus may be seen putting his or her thumb into the mouth and sucking on it.
Weeks 33-40
During this final stage of the pregnancy, any remaining lanugo is usually shed, and the nails grow slightly beyond the tips of the fingers and toes. The fetus grows larger and stronger, but there are no major qualitative changes in his anatomy. The brain, however, continues to develop at a rapid pace. According to "What Babies Can Do," when the fetus is born, he will immediately be capable of seeing and hearing the surrounding environment and of making some rudimentary sense of it.
