Over the course of a normal pregnancy, a single fertilized egg cell develops into a full-term fetus. The process usually lasts between 38 and 42 weeks and consists of a large number of stages that occur in a particular order at specific times. Scientists have characterized this development in tremendous detail, most of which cannot fit within this brief article. Several of the most notable milestones will be summarized here.
How Old Is The Fetus?
Doctors use two different systems for describing ages during pregnancy. Sometimes, doctors refer to the amount of time that has passed since the start of the mother's last menstrual cycle prior to pregnancy. This is known as the gestational age. The egg is not fertilized, however, until approximately two weeks later. Since the exact time that the fertilization takes place can only be inferred, gestational age is most often used. This article will refer to gestational ages. These can be converted to post-fertilization ages, of course, by simply subtracting two weeks.
The Stage of the Zygote: 14 to 19 Days
Once the mother's egg is fertilized by the father's sperm, it is referred to as a zygote. This single cell contains half of the mother's genetic information and half of the fathers, combined to form a new, complete set of DNA. Fraternal twins result if two eggs are released and fertilized in a single menstrual cycle, forming two zygotes that are no more related than any two siblings. Identical twins develop from a single zygote when it divides to form two separate bodies during the embryonic stage.
The Stage of the Blastocyst: 19 to 24 Days
The zygote immediately begins to divide and replicate itself, forming a ball of several hundred cells called the blastocyst. After it travels to the end of one of the mother's fallopian tubes, it embeds itself into the wall of the uterus. Most of the blastocyst is only one cell thick. One segment of it, however, is three to four cells thick. The thick region of the blastocyst will develop into the embryo and eventually the fetus. The thin portion of the blastocyst develops into the placenta, an organ that provides a connection to the mother and helps to regulate development. Part of the placenta forms a thin, protective, fluid-filled membrane called the amniotic sac. The portion that will become the embryo floats within this sac, connected to the placenta via the umbilical cord.
The Stage of the Embryo: Four to Nine Weeks
During this stage of rapid development, the foundations of all of the primary body parts form. The spinal cord and brain appear and begin producing electrical activity. Once the muscles form, this activity will cause the body to move. The arms and legs sprout as well. A few days later, fingers and toes begin to grow at the ends of these limbs. The heart forms and begins to beat, causing blood to circulate through the embryo's emerging arteries and veins. Birth defects are sometimes caused by exposure to harmful teratogens---toxins, infections or radiation. Such exposures can be harmful at any stage of a pregnancy, but the embryo is especially susceptible to these problems. It is thus especially important for an expectant mother to avoid teratogens during this stage.
The Stage of the Fetus: 10 to 39 Weeks
The fetal stage begins at 10 weeks. A fetus possesses all of the primary body parts that will be present at the time of birth. The development that occurs during this period provides finishing details for the foundations laid during the embryonic stage. At 11 weeks, the eyelids of the fetus close. The eyelids provide a protective cover as the eyes develop further. When they eyes do open, around 30 weeks, the fetus will begin to periodically blink. The fetus's genitals become differentiated around week 18, making it possible to tell whether the fetus is male or female in a typical ultrasound scan. At around 19 weeks, there is evidence that a fetus can hear, according to the National Institutes of Health. This is based on findings that when a loud sound is emitted outside the womb, the fetus usually exhibits an abrupt change in his or her level of activity. According to the Merck Online Medical Manual, the fetus grows large enough to fill the amniotic sac at around 22 weeks. The fetus moves around quite a bit prior to this, but, as he or she grows larger and stronger at this stage, the expectant mother will increasingly feel kicking and pushing movements. According to the National Institutes of Health, week 26 is the earliest stage at which a fetus has a chance of surviving outside the womb. This is because at this stage, the lungs of the fetus develop the ability to perform gas exchange---deriving oxygen from air and expelling carbon dioxide. Ordinarily, of course, the fetal lungs do not do this, since all oxygen and nutrients are delivered via the umbilical cord.
Birth: Approximately 40 weeks
In the final weeks of the pregnancy, the body of the fetus grows larger and stronger, but there are no major, qualitative developments. There is evidence, however, that the brain of the fetus continues to develop in important ways. According to the What Babies Can Do DVD, when the fetus is born, he or she will emerge with the ability to see, hear and make some sense of the surrounding environment, starting from the moment of birth.
