The human fetus develops in the mother's womb over the course of approximately 38 weeks. The development is remarkably complex and follows a precisely-timed set of steps. The growth stages have been characterized in great detail, but several of the most salient milestones can be summarized rather simply.
Growth that Occurs Prior to the Fetal Stage
A fertilized egg is not yet a fetus. The term fetus is used only after the eighth week of the pregnancy. Initially, the single, fertilized egg cell is referred to as a zygote. After fertilization in one of the mother's fallopian tubes, the zygote travels to and embeds itself in the wall of the uterus. During this time, the zygote rapidly divides, forming a ball of several hundred cells called the blastocyst. The blastocyst continues to grow and segments itself into two parts. One part becomes the placenta, an organ that provides a connection to the mother and helps to regulate development. The other part of the blastocyst becomes the embryo, which eventually becomes the fetus approximately eight weeks after fertilization.
Fetal Growth During Week 8
During the eighth week of the pregnancy, the fetal stage begins. The developing fetus has all of its major body parts intact. The brain and spinal cord are present and produce electrical activity that causes the fetus body to move from time to time. The heart is present and beats, causing blood to circulate through the body. The arms and legs are formed, with fingers and toes as well. The fetus floats within a fluid-filled membrane---part of the placenta---called the amniotic sac. An umbilical cord connects the fetus to the placenta and provides all of the oxygen and nutrients that the fetus requires to support continued development.
Fetal Growth During Weeks 9 to12
During the ninth week of development, the eyelids of the fetus close, providing protection to the eyes as they develop until week 28. The genitals of the fetus differentiate between weeks 9 and 12, according to the National Institutes of Health. If an ultrasound scan is performed beyond this stage, it is usually possible to determine if the fetus is male or female. The fetus has all of its major body parts at this stage, but their relative size is very different from those of an adult or even a newborn baby. For instance, the head makes up fully half of the overall height of the fetus.
Fetal Growth During Weeks 13 to 19
A very fine hair spouts during week 13 of fetal growth. This lanugo eventually covers the entire body of the fetus. It is typically shed by the time the baby is born, but some is occasionally still present at the time of birth, especially if the birth is premature. Several studies have shown that the fetus will respond to sound by week 17, according to the National Institutes of Health. If a loud sound is made outside the womb, the fetus will often exhibit a correlated change in his level of movement. These results suggest that the fetus can hear at this stage. Also during this time, the movements of the fetus become more coordinated. For instance, the fetus will often exhibit sucking movements with the mouth and tongue that cause amniotic fluid to be ingested.
Fetal Growth During Weeks 20 to 23
Around week 20, according to the Merck Online Medical Manual, most fetuses grow large enough to fill the amniotic sac. Beyond this stage, the movements of the fetus will be felt by the expectant mother. These kicking and pushing sensations will increase in frequency for the rest of the pregnancy. At this stage of the fetus's growth, her heartbeat can first be heard with a stethoscope. It is worth noting, however, that this can be challenging, since the position of the fetus heart changes as she moves, and the heartbeat of the mother is much louder.
Fetal Growth During Week 24
According to the Merck Online Medical Manual, the lungs of the fetus develop the ability to accomplish gas exchange at week 24, enabling them to absorb oxygen and expel carbon dioxide from air. The lungs won't normally do this, since the fetus obtains oxygen and expels carbon dioxide via blood circulation through the umbilical cord. However, if the fetus is born prematurely beyond this point in development, he will have some chance of survival because of the onset of this lung function.
Fetal Growth During Weeks 25 to 32
The brain and nervous system of the developing fetus become increasingly active during this stage of growth. The eyes open and blink periodically. The movements of the fetus also increase in complexity and coordination. In many instances, during ultrasound scans conducted beyond this stage, the fetus can be seen putting the thumb into the mouth and sucking on it. The lungs begin to expand and contract in breathing-like motions at this stage as well. As they do, the fetus pulls in and expels amniotic fluid from the lungs.
Fetal Growth During Weeks 33 to 40
The final weeks or growth are associated with increased size and strength of the fetus, but there are no significant, qualitative changes in her body. The lanugo is typically shed during this time period. The fetus fingernails and toenails typically reach the ends of the fingers and toes and grow slightly beyond their tips. Around 38 weeks, the fetus will usually be born. According to the "What Babies Can Do" DVD, the fetus will be capable of seeing, hearing, and mentally processing the surrounding world as soon as he is born.
