When the mother's egg is initially fertilized by the father's sperm, it is referred to as a zygote. This single cell develops into a ball of cells called the blastocyst, and then an embryo over the following two weeks. During week eight, it becomes a fetus. Over the following 30 to 34 weeks, continued development prepares the fetus for life outside the womb. This process follows a set of specifically ordered and timed steps that have been characterized in great detail by developmental scientists.
Fetus Development in Week Eight
As the fetal stage of development begins, the fetus floats within a fluid-filled membrane call the amniotic sac, attached by an umbilical cord to the placenta, an organ that provides a connection to the mother and helps to regulate development. All of the nutrients needed to support the development of the fetus come from the mother's blood stream, through the placenta, via the umbilical cord. During the embryonic stage that leads up to this point in the pregnancy, the foundations of all of the body parts and organs form. During the fetal stage of development, these parts grow in both size and complexity. Even at week eight, however, the heart is already present and beating, causing blood to course through the veins and arteries of the fetus. The brain and spinal cord are both already formed and produce recordable electrical activity. The arms and legs are also present, and the fingers and toes are growing at their ends.
Fetus Development in Weeks Nine to 12
In the ninth week of the pregnancy, the eyelids of the fetus close and stay closed for the next 19 weeks, providing a protective cover for the eyes during this critical stage of development. The genitals of the fetus will differentiate at this stage, according to the National Institutes of Health. Any ultrasound scan that is performed after this point in fetus development may reveal whether the fetus is male or female.
Fetus Development in Weeks 13 to 19
A very fine hair called lanugo appears at the beginning of this stage. Eventually it covers the entire body of the fetus. In most cases, it is shed before birth, but some may remain, especially of the birth is premature. In week 17, according to the National Institutes of Health, the fetus begins to respond to sound. If a loud noise is made outside the womb, the fetus will often exhibit an abrupt change in activity level. Results such as this suggest that 17-week-old fetuses can hear. Throughout the pregnancy, the actions of the fetus become increasingly coordinated. This is apparent in the nineteenth week, as the fetus begins making sucking motions with the mouth and tongue, pulling amniotic fluid into the mouth and then expelling it.
Fetus Development in Weeks 20 to 23
Around week 20, the expectant mother will begin to feel the kicking and pushing movements of the fetus. While the fetus starts moving as early as week eight, only at this point will the mother feel it. This is because the fetus grows large enough to fill the amniotic sac in week 20, according to the Merck Online Medical Manual. The movements will be felt more and more frequently over the rest of the pregnancy, especially in time periods after the mother has consumed something sweet such as fruit or juice. This is also the stage of development when the heartbeat of the fetus can first be heart via a stethoscope placed on the mother's belly. Expectant parents shouldn't worry if this doesn't work every time, however, since the heart position changes with the movement of the fetus, and the heartbeat of the mother is always much louder.
Fetus Development in Week 24
After week 24, the developing fetus has some chance of survival if he or she is born prematurely. According to the Merck Online Medical Manual, this is because of the onset of the ability of the lungs to perform gas exchange---absorbing oxygen from and expelling carbon dioxide into the air. In the normal course of the pregnancy, however, the lungs are not needed for this, since all oxygen needed by the fetus is present in the blood from the mother that circulates through the umbilical cord.
Fetus Development in Weeks 25 to 32
In week 28, the eyelids of the fetus open again, and thereafter blink open and closed from time to time. They will stay closed during the hours when the fetus sleeps. The lungs become stronger and more coordinated in this stage as they pull amniotic fluid in and expel it in breathing-like motions. The movement of the fetus continues to grow more coordinated. For instance, an ultrasound scan performed from this stage onward will often show the fetus putting the thumb into the mouth and sucking it.
Fetus Development in Weeks 33 to 40
These are the final weeks of a normal pregnancy. During this time, a few growth milestones are reached. For instance, the lanugo is shed, and the fingernails and toenails grow slightly beyond the fingers and toes. However, most of the physical developments are changes are in size, not significant changes in anatomy. The brain of the fetus, however, continues to develop rapidly. According to the What Babies Can Do DVD, even at the moment that the fetus is born, he will be able to see, hear and make some sense of the surrounding environment.
