When the mother's egg is fertilized by sperm, it forms a single cell called a zygote. The zygote grows into a ball of cells called a blastocyst and embeds itself into the wall of the mother's uterus. Over the subsequent two weeks, it becomes an embryo, and by week eight of the pregnancy the fetal stage of development begins. Fetal development progresses through a precisely ordered and timed set of stages.
Week 8
During the embryonic stage that precedes the fetal stage, all of the main body parts and organs form. By week eight, the heart is present and beating, causing blood to circulate through the growing arteries and veins of the fetus. The spinal cord and brain are also present and produce recordable electrical activity. These signals will cause the growing muscles to contract from time to time, in turn causing the fetus to move. The arms and legs are both present as well, with fingers and toes growing at their ends.
During the remaining weeks of the pregnancy, all of the body parts of the fetus will grow in size, complexity and coordination. The body of the fetus floats within a fluid-filled, protective membrane called the amniotic sac. The fetus is attached by an umbilical cord to the placenta, an organ that provides a connection to the mother and serves to regulate development. Blood circulates from the mother's blood stream through the placenta and then through the umbilical cord, bringing all of the nutrients and oxygen needed to support development.
Weeks 9-12
At the start of this stage of the development, the eyes of the fetus close and remain closed until the 28th week of the pregnancy. The eyelids will provide a protective cover for the eyes as they develop during this extended, critical period. By week 12, the genitals of the fetus differentiate. If an ultrasound scan is performed any time after the 12th week, it may be possible to determine whether the fetus is male or female.
Weeks 13-19
A very fine hair, called lanugo, sprouts at the beginning of this stage. It initially grows on the head but eventually covers the entire body of the fetus. Typically, the lanugo will be shed prior to birth, but it is not unusual to see some on a newborn, especially if the birth is premature. According to the National Institutes of Health, the fetus begins to respond to sound during this stage, in the 17th week of the pregnancy. If a loud noise is made near the mother, the fetus will often exhibit an abrupt change in his level of activity. This type of result is usually interpreted as evidence that 17-week-old fetuses can hear. The fetus continues to perform increasingly complex behaviors during this stage. By week 19, the fetus will start to exhibit sucking motions with the mouth and tongue. As he does so, the fetus will pull amniotic fluid into his mouth and then expel it.
Weeks 20-23
This is the stage during which expectant mothers will first begin to feel the kicking and pushing movements of the developing fetus. According to the Merck Online Medical Manual, the fetus grows to fill the amniotic sac by week 20 of the pregnancy. The fetus has been moving at this point since the 8th week of the pregnancy, but the size of the fetus will now make it possible to feel the movements. These movements will be felt with increasing frequency for the remainder of the pregnancy. The movements will be especially salient during periods after the mother has consumed something sweet, such as fruit juice. This is also the first stage during which the heartbeat of the fetus can be heard using a stethoscope. Expectant parents should not be alarmed, however, if this does not always happen. As the fetus moves to different positions within the womb, the position of the heart is unpredictable, and the heartbeat of the mother is always much louder.
Week 24
During week 24 of the pregnancy, according to the Merck Online Medical Manual, the lungs of the fetus develop the ability to perform gas exchange, driving oxygen from air and expelling carbon dioxide into it. Under normal circumstances, the fetus will derive all of its needed oxygen from the bloodstream of the mother, via the umbilical cord. If the infant is born prematurely, however, the onset of this lung function will provide the fetus with some chance of survival outside the womb.
Weeks 25-32
The eyes of the fetus open again during this stage, and will blink open and closed periodically, just as they will after the fetus is born. The exception to this is when the fetus sleeps, during that time, the eyes will remain closed for several hours.
The lungs continue to develop in strength and coordination as well, making breathing-like movements that pull amniotic fluid in and expel it. If an ultrasound is performed during this stage, even more complex behaviors may be seen. For instance, the fetus periodically puts her thumb into her mouth and sucks on it.
Weeks 33-42
During the final weeks of the pregnancy, the fetus grows larger and stronger, but there are few major anatomical changes. The fingernails and toenails grow to reach the ends of the fingers and toes, and the lanugo is shed, but the basic anatomy of the fetus remains largely the same. The brain of the fetus does continue to develop at a rapid pace during this stage, however.
When the fetus is born, usually around 37 weeks into the pregnancy, he will immediately be able to see, hear and make some sense of the surrounding world, according to the What Babies Can Do DVD. For instance, a newborn will be able to track with his eyes and will exhibit a preference to look at human faces from the moment of birth, even though he has spent the past 9 months developing in complete darkness.
