A fetus develops according to a precisely ordered and timed set of stages during a pregnancy, which on average lasts between 38 to 42 weeks. When the mother's egg is fertilized---usually in one of the mother's fallopian tubes---by the father's sperm, it becomes a zygote. Over the next two weeks, the zygote develops into a ball of cells called the blastocyst and then into an embryo. By week eight of the pregnancy, the fetal stage begins.
The Start of the Fetal Stage: Week 8
At the start of this stage, the fetus already possesses all of its primary body parts and organs. The remainder of the pregnancy is mostly about finishing the development of these parts, increasing their size, complexity and coordination. The heart is already present and beating, causing blood to circulate through the arteries and veins that are rapidly developing within the fetus. The fetus also has arms, legs, fingers and toes. The brain and the spinal cord are also present, and both produce recordable electrical activity. These neural impulses cause the growing muscles of the fetus to contract periodically, and in so doing cause the fetus to move. The body of the fetus floats within a protective, fluid-filled membrane called the amniotic sac. An umbilical cord provides a connection between the fetus and the placenta, an organ that connects to the mother's bloodstream and helps to regulate development. All of the nutrients that are needed to support development are delivered from the mother to the fetus via the placenta and the umbilical cord.
Eyes Close and Genitals Differentiate: Weeks 9 to 12
Around week nine of the pregnancy, the eyes of the fetus close and remain closed for about 19 weeks. During this period of critical eye development, the eyelids provide a protective covering. By week 12, the genitals of the fetus will differentiate. If an ultrasound scan is performed from this point onward, it may be possible to determine whether the fetus is male or female.
The Onset of Hearing, Lanugo and Sucking Behaviors: Weeks 13 to 19
At the start of this stage, a very fine hair called lanugo sprouts on the head of the fetus. Eventually, the lanugo grows on every part of the skin of the fetus. By the time the fetus is born, this lanugo will typically be shed, but some may still be visible, especially if the birth is premature. By the middle of this stage, around week 17, the fetus will begin to respond to sound, according to the National Institutes of Health. If a loud sound is made near the mother, the fetus will often abruptly alter his level of activity. This type of result suggests that the 17-week-old fetus can hear. From this stage onward, the actions of the fetus grow progressively more complex and coordinated. In this stage, the best example of this is the onset of sucking motions with the mouth and tongue. As the fetus makes these motions, amniotic fluid is pulled into the mouth and expelled.
The Onset of Kicking and Audible Heartbeats: Weeks 20 to 23
According to the Merck Online Medical Manual, the fetus grows large enough to fill the amniotic sac in the 20th week of the pregnancy. As this happens, the expectant mother will begin to feel kicking and pushing movements. While the fetus has been moving for more than 12 weeks at this point, it is only now that the mother will feel them. The motions will be felt with increasing frequency for the remainder of the pregnancy, especially during periods after the mother has consumed something sweet such as orange juice. Also during this stage, the heartbeat of the fetus can first be heard using a stethoscope. However, if expectant parents try this and cannot hear the heartbeat, there is no cause for alarm, however. The position of the fetus heart is unpredictable as the fetus shifts within the womb, and the mother's heartbeat is much louder.
The Onset of Lung Function and Associated Viability: Week 24
According to the Merck Online Medical Manual, at week 24, the lungs of the fetus develop the ability to perform gas exchange---transferring oxygen from air into the bloodstream and carbon dioxide from the blood to the air. For most pregnancies, the fetal lungs won't actually perform this function for many more weeks. The fetus is submerged within amniotic fluid, and all oxygen and nutrients are delivered from the mother's bloodstream via the umbilical cord. If the fetus is born prematurely beyond week 24, however, the onset of this lung function will provide him with some chance of survival outside the womb.
Breathing Practice, Blinking and Thumb Sucking: Weeks 25 to 32
During this stage of development, the lungs of the fetus can be seen to make breathing-like movements if an ultrasound scan is performed. As the lungs expand and contract, amniotic fluid is pulled into the lungs and then expelled. Other coordinated behaviors emerge during this stage as well. For instance, the fetus will periodically put his thumb into his mouth and then suck on it. Near the middle of this stage, the eyes of the fetus finally open again. After they do, the eyes will periodically blink open and closed, just as they will after the fetus is born. When the fetus sleeps, however, the eyes will remain closed for several hours at a time.
Finishing Touches: Weeks 33 to 42
This is the final stage of the pregnancy before the fetus is born. The body grows larger and stronger, and a few notable physical developments occur. The lanugo is shed and the fingernails and toenails grow to and slightly beyond the ends of the fingers and toes. Other than the changes in size, however, there are few significant changes in the anatomy of the fetus. The brain of the fetus continues to develop rapidly throughout this stage. According to the What Babies Can Do DVD, the fetus will be born with a large set of perceptual and cognitive abilities. For instance, as soon as the fetus is born, he will immediately be able to see, hear and make some sense of the surrounding world. A newborn will follow moving objects with eyes and show a visual preference for human faces, even though the fetus has spent the entire pregnancy in the dark.
