During the 38 to 42 weeks of a typical pregnancy, a fetus develops in the mother's womb. The mother's egg is fertilized by the father's sperm to produce a single cell called the zygote. This develops into a ball of several hundred cells called the blastocyst that embeds itself in the wall of the uterus, where it develops into an embryo. By week eight, the fetal stage begins.
Week 8--Beginning of the Fetal Stage
During the embryonic stage of development, all the major body parts and organs are formed. During the fetal stage, these parts grow in size, complexity and coordination to prepare the fetus for life outside the womb. The heart is already present at the start of week eight. It beats, causing blood to circulate through the developing arteries and veins. The brain and spinal cord are also formed and produce recordable electrical activity that causes the muscles to contract and the fetus to move from time to time. The arms and legs are present, and fingers and toes grow at their ends. The body of the fetus is attached to the mother via the umbilical cord, which brings nutrient- and oxygen-rich blood from the mother's bloodstream via the placenta. The fetus floats within a protective, fluid-filled membrane called the amniotic sac.
Weeks 9 to 12--Eyes Close and Genitals Differentiate
During this stage of development, the eyes close and will remain closed for fully 19 weeks. The eyelids provide a protective cover for the eyes as they undergo extensive development behind them. Also in this stage, the genitals of the fetus differentiate, such that an ultrasound scan may enable a determination of whether the fetus is male or female, either during or at any time beyond this stage.
Weeks 13 to 19--Onset of Hearing, Lanugo and Sucking Behaviors
A very fine hair, called lanugo, begins growing near the start of this stage. Eventually, it covers the entire body. Most of the lanugo will be shed shortly before the fetus is born, but it is not unusual to see some still on the body at that time, especially if the birth is premature. Also during this stage, around 17 weeks into the pregnancy, the fetus becomes able to hear, according to the National Institutes of Health. This is based on results of studies in which a loud noise is made near the mother. When this happens, the fetus often abruptly changes his level of activity, suggesting that the sound has been heard. The actions of the fetus grow progressively more coordinated throughout the pregnancy. During this stage of development, for example, the fetus will begin making sucking motions with his mouth and tongue, pulling amniotic fluid into the mouth in the process.
Weeks 20 to 23--Onset of Kicking and Audible Heartbeats
The fetus starts moving as early as the eighth week of pregnancy but not until the 20th week will expectant mothers begin to feel these movements. According to the Merck Online Medical Manual, this is because in week 20, the fetus grows to completely fill the amniotic sac. The kicking and pushing movements will be felt more and more frequently over the remainder of the pregnancy. A period of high activity will often occur after the mother has consumed something sweet, such as fruit juice. This is also the stage during which the heartbeats of the fetus can first be heard with a stethoscope placed on the mother's belly. It is important to note that this won't always work, however, because the moving fetus makes the position of the heart unpredictable. It is also the case that the mother's heartbeat is always substantially louder.
Week 24--Onset of Lung Function and Associated Viability
Week 24 marks an important developmental milestone. Beyond this age, the fetus has some chance of survival outside the womb if she is born prematurely. According to the Merck Online Medical Manual, this is due to the onset of the ability of the lungs to perform gas exchange---absorbing oxygen from the air into the bloodstream and emitting carbon dioxide from the bloodstream to the air. During most pregnancies, the lungs do not perform this function this early, however. The fetus remains continually submerged in amniotic fluid, and the task of delivering oxygen to and carbon dioxide away from the developing fetus is accomplished via the umbilical cord's connection to the mother's bloodstream.
Weeks 25 to 32--Breathing Practice, Blinking and Thumb Sucking
As the fetus continues to develop during this stage, the lungs become stronger and more coordinated. A fetus frequently makes breathing-like movements, pulling amniotic fluid into and expelling it from the lungs. Other behaviors also become more coordinated. For instance, the fetus will periodically put the thumb into his mouth and suck on it. In week 28, the eyes finally open again and begin to blink periodically, just as they will after the fetus is born. When the fetus sleeps, the eyes will remain closed for as many as several hours, however.
Weeks 33 to 42--Finishing Touches
During the last four to nine weeks of pregnancy, the fetus grows larger and stronger. Dome physical milestones are achieved, such as shedding of the lanugo and the growth of fingernails and toenails to the ends of fingers and toes. However, no major qualitative changes occur in the anatomical structure of the fetus. The brain does continue to develop at a rapid pace. According to the What Babies Can Do DVD, when the fetus is born---usually some time after 37 weeks---he will immediately be able to see, hear and make some sense of the surrounding environment. For instance, a newborn will follow things with his eyes and exhibit a preference for human faces.
