A fetus develops over the course of a 38- to 42-week pregnancy, from a single, fertilized egg cell---called a zygote---into a full-term fetus, ready for life outside the womb. After being fertilized, the zygote divides and replicates itself rapidly, forming a ball of several hundred cells called a blastocyst. The blastocyst embeds itself in the wall of the mother's uterus and develops into an embryo over the next two weeks. By week 8, the fetal stage begins.
Week 8
By the beginning of this stage, the fetus already possesses all of its primary body parts and organs. The heart beats, causing blood to circulate through the body. The arms and legs are present, with the fingers and toes growing at their ends. The brain and spinal cord are both apparent as well, and both produce electrical activity. As the muscles develop, the activity of the brain will cause the fetus to move from time to time. In essence, the fetus already possesses rudimentary versions of all of the primary body parts. The rest of the pregnancy will be about refining those parts, increasing both their complexity and coordination. The fetus itself floats within a fluid-filled, translucent membrane called the amiotic sac. An umbilical cord connects the fetus to the placenta, an organ that provides a connection to the mother and helps to regulate development. All of the nutrients and oxygen needed to support development come to the fetus via the placenta and the umbilical cord.
Weeks 9 to 12
At the start of this phase of development, the eyelids of the fetus close and remain closed for the next 19 weeks. The eyelids will provide a protective cover for the eyes during this extended, critical period of development. Around the 12th week, the genitals of the fetus will differentiate. When ultrasound scans are performed from this stage on, it may be possible to determine if the fetus is male or female.
Weeks 13 to 19
At the beginning of this phase of development, the fetus will begin to grow a very fine hair called lanugo. Over the course of a few weeks, this lanugo grows to cover the entire body of the fetus. In the course of a normal pregnancy, the fine hair will be shed prior to birth, but it is not uncommon for some to remain at that time, especially if the birth is premature. Around week 17, according to the National Institutes of Health, the fetus will begin to respond to sound. If a loud noise is made outside the womb, the fetus will often abruptly change his level of activity. This type of result suggests that beyond 17 weeks, the fetus can hear. In ultrasound scans performed beyond this stage, it is often possible to see the fetus performing coordinated sucking actions with the mouth and tongue. As he does so, amniotic fluid will be pulled into the mouth.
Weeks 20 to 23
This is the stage of development when expectant mothers will first begin to feel the kicking and pushing movements of the fetus. Although the fetus has been moving since week 8 of the pregnancy, it will now be felt because, according to the Merck Online Medical Manual, this is when the fetus grows large enough to fill the amniotic sac. The fetal shifts in position, kicks and pushes will be felt with increasing frequency for the remainder of the pregnancy, especially during periods after eating something sweet such as fruit. This is also the first stage in which the heartbeat of the fetus can be heard using a stethoscope placed on the mother's belly. If this doesn't work every time, however, expectant parents should not be alarmed. As the fetus moves, the position of her heart can be unpredictable, and the heartbeat of the mother is always much louder.
Week 24
According to the Merck Online Medical Manual, the lungs of the fetus develop the ability to perform gas exchange during week 24 of the pregnancy. The lungs become capable of transferring oxygen from air to the bloodstream and carbon dioxide from the bloodstream to the air. Under normal circumstances, the lungs will not perform gas exchange, of course. The fetus is submerged in amniotic fluid and all of the oxygen needed is delivered from the mother's bloodstream via the umbilical cord. This is a critical milestone, however. Any infant born prematurely beyond this age will have some chance of survival because of this lung function.
Weeks 25 to 32
The lungs continue to develop during this phase, improving their strength and coordination as the fetus makes breathing-like movements. As he does so, amniotic fluid is pulled into the lungs and then expelled. If an ultrasound scan is performed at this stage, it will often be possible to see a new, coordinated behavior that will continue for the remainder of the pregnancy and even after birth. The fetus will put the thumb into the mouth and then suck on it. Also during this stage, around week 28, the fetus will open his eyes again and thereafter periodically blink them open and closed. The exception to this will be when the fetus sleeps, when the eyes will remain closed for as many as several hours.
Weeks 33 to 42
In these final weeks of the pregnancy, there are a few milestones of physical growth. For instance, the fingernails and toenails grow to reach the ends of the fingers and toes, and most of the lanugo is shed. Other than increasing size and strength, however, there are no major anatomical changes. The brain of the fetus, however, continues to develop rapidly during this time. According to the What Babies Can Do DVD, when the fetus is born, she will immediately be able to see, hear and make some sense of the surrounding world.
