After an egg is fertilized, it implants itself in the mother's uterus and begins the process of developing from a single cell to a full-term fetus over the course of approximately 38 weeks. Fetal development follows a consistent set of stage-like steps.
Stages that Lead Up to Fetal Development
When the egg is first fertilized, it is called a zygote---not yet a fetus. It implants in the lining of the mother's uterus and grows into a ball of cells called the blastocyst. The blastocyst continues to grow and divides into two parts. One part becomes the placenta, an organ that serves as a connection to the mother and helps to regulate fetal development. The other part becomes the embryo, which develops into the fetus.
Week 8
By the start of the fetal stage of development, the placenta has formed a thin, fluid-filled membrane called the amniotic sack. The fetus floats in this fluid, attached to the placenta via the umbilical cord. Also by the start of the fetal stage, the main parts of the body are present. The spinal cord and brain can be identified, as can the heart, which beats and pumps blood. The fetus has arms, legs, fingers and toes. The fetal brain is active enough that electrical impulses can be recorded. This activity stimulates the muscles of the fetus, causing its body to move.
Weeks 9 to 12
During this stage, the eyelids close, protecting the developing eyes, and stay closed until week 28. According to the National Institutes of Health, the genitals become differentiated near the start of this fetal stage, making it possible for a doctor to determine if the fetus is male or female. The head of the fetus at this stage makes up about 50 percent of its total height.
Weeks 13 to19
In this stage, the fetus grows a temporary coat of very fine hair, called lanugo. It will typically disappear by the time the fetus reaches full-term, but some may remain at birth, especially for pre-term births. According to the National Institutes of Health, the fetus develops the ability to hear around week 17 and will respond to sound that is transmitted from the outside world through the womb. The fetal movements become more coordinated in this stage. For instance, the fetus starts to make sucking motions with its mouth.
Weeks 20-23
Around week 20, eyebrows and eyelashes appear and nails start growing on the fetus' fingers and toes. According to the Merck Online Medical Manual, as the fetus becomes large enough to fill the amniotic sack, the mother will feel periodic movements. The heartbeat of the fetus can first be heard with a stethoscope during this stage, although it should be noted that this requires some practice, as the mother's heartbeat is still far louder.
Week 24
During week 24, fingerprints and footprints form in the fetus' skin surface. Abrupt sounds or movements will activate the fetus's startle reflex, which starts to function at this age. Also around week 24, the lungs develop some ability to derive oxygen from air. Under normal circumstances, all of the fetus's oxygen is obtained from the mother's bloodstream, via the umbilical cord. According to the Merck Online Medical Manual, if the fetus is born after 24 weeks, however, the lungs will be able to function to some degree.
Weeks 25-32
During this stage, the fetus' nervous system undergoes extensive and rapid development. The eyes open and periodically blink. The lungs move in repetitive breathing motions, pulling amniotic fluid into the lungs and then expelling it. The arms and legs move under the fetus' control. The fetus occasionally sucks on a thumb.
Weeks 33-40
During this stage, the fetus continues to grow bigger, but most large anatomical changes are finished. The lanugo disappears. The fingernails finish growing out to the ends of the fingers. During this final stage, the mother's labor will begin and a newborn baby will be born.
