A typical pregnancy lasts between 38 and 42 weeks, during which time a fertilized egg cell develops into a full-term fetus, ready to be born. The developmental process progresses through a detailed and precisely timed set of stages during the pregnancy. Although a tremendous amount is known about the development of the fetus, pregnancy itself can be easily described by six stages.
Gestational Age vs. Post-Fertilization Age
It is worth noting that doctors refer to the age of a developing fetus in two very different ways, which can lead to some confusion. In some cases, doctors refer to the amount of time that has passed since the first day of the mother's last menstrual cycle. This is known as the gestational age. The moment at which the egg is fertilized does not occur until about two weeks later, however. Since this moment can only be inferred, the gestational age is most often used. In this article, development is described in terms of post-fertilization age, but the gestational age can be calculated by simply adding two weeks to each post-fertilization age.
0--5 Days: Zygote Stage
At the moment that a mother's egg is fertilized by the father's sperm, the single resulting cell is referred to as a zygote. Half of the mother's and half of the father's DNA are combined, resulting in a single, complete set of genetic material. In some cases, two eggs are released and fertilized in the same menstrual cycle, resulting in fraternal twins. Identical twins result when a single egg is fertilized, but later divides into two bodies during the embryonic stage of development.
5--10 Days: Blastocyst Stage
As the zygote travels down one of the mother's fallopian tubes and into her uterus, it rapidly divides and replicates itself, forming a ball of cells known as the blastocyst. The blastocyst embeds itself into the wall of the uterus and forms two separate segments. One part will become 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 itself and eventually the fetus. Part of the placenta forms a translucent, fluid filled membrane called the amniotic sac. The embryo floats within this protective enclosure and is attached to the placenta by an umbilical cord.
2--7 Weeks: Embryonic Stage
In this developmental stage of the pregnancy, all of the main parts of the body emerge, albeit in a rudimentary form. The spinal cord and brain emerge and begin to produce electrical activity that causes the body to move from time to time. The heart is also formed and begins to beat, causing blood to circulate through the developing embryo's body. The arms and legs sprout and grow, followed shortly thereafter by fingers and toes. Birth defects are sometimes caused by exposure to substances called teratogens---toxins, infections, or radiation--during this stage. Expectant mothers should always avoid these, but this is especially true during the rapid development of the embryonic stage.
8-37 Weeks: Fetal Stage
After eight weeks, the embryo is referred to as a fetus. A wide range of additional developments take place that refine the body parts that emerge by the end of the embryonic stage. Around nine weeks, the eyelids of the fetus close, providing a protective cover for the developing eyes until week 28, when they open again and begin to periodically blink. Around week 16, the genitals of the fetus become differentiated. If an ultrasound scan is performed after this age, it will usually be possible to determine if the fetus is male or female. According to the National Institutes of Mental Health, infants begin to respond to sound in week 17. If a loud noise is made outside the womb, the fetus will exhibit an abrupt change in his or her level of activity. This suggests that after 17 weeks, fetuses can hear. During week 20, according to the Merck Online Medical Manual, the fetus grows large enough to fill the amniotic sac. After this time, expectant mothers will feel kicking and pushing movements. According to the National Institutes of Health, fetal lungs become capable of deriving oxygen from and expelling carbon dioxide into air at around 24 weeks. Typically, the fetal lungs do not need to do this, since the fetus obtains all necessary oxygen and nutrients via the umbilical cord. If the fetus is born prematurely after this stage, however, the lung function will provide the fetus with some chance of survival.
38--42 Weeks: Full-Term Birth
In this final stage of the pregnancy, the fetus achieves full-term development and will typically be born. The fetus continues to grow larger and stronger during these weeks, but there are no major, qualitative changes in the fetus beyond 37 weeks. Brain development continues at a rapid pace, however. According to the What Babies Can Do DVD, the fetus will emerge with the ability to see, hear, and make some sense of the surrounding world from the moment that he or she is born.
