During the 37 to 42 weeks of a typical pregnancy, a single fertilized egg cell develops into a full-term baby with all of the many functioning parts that make up a human body. This developmental process is amazingly complex and has been the subject of many multivolume textbooks. Here are the basic stages of development that occur inside the mother's womb.
Gestational Age Versus Post-Fertilization Age
Doctors often calculate the age of a developing fetus by determining how much time has passed since the first day of the mother's last menstrual cycle prior to pregnancy. This is referred to as the gestational age of the fetus. The actual fertilization of the egg usually takes place about two weeks after this date. Because the age based on the moment of fertilization can only be inferred, the gestational age is usually used. This article will describe development in terms of the post-fertilization age.
Zygote Stage (Zero to Five Days)
A fertilized egg is referred to as a zygote. Half of the mother's DNA combines with half of the father's DNA to form a complete set of genetic material. Fraternal twins are produced if two zygotes form at the same time, that is, if two eggs are released, both eggs are fertilized, and both zygotes develop. Identical twins are produced from a single zygote that splits into two bodies during the embryonic stage.
Blastocyst Stage (Five to 10 Days)
The single zygote cell rapidly divides and replicates itself as it travels down the fallopian tube and into the uterus. By the time it implants itself into the wall of the mother's uterus, it will have become a ball of several hundred cells. This is called the blastocyst. As it continues to grow, the blastocyst segregates itself into two groups of cells. One group becomes the embryo; the other becomes the placenta, an organ that serves as a connection to the mother and helps to regulate development.
Embryonic Stage (Two to Seven Weeks)
During the embryonic stage, the main parts of the body emerge. The spinal cord and the brain develop. The heart takes shape and begins to beat. The arms and legs form as well. Near the end of this stage, fingers and toes will begin to form on the ends of the limbs. As prenatal development progresses, birth defects are sometimes caused by teratogens--toxins, infections or radiation. The embryonic stage is a period of high susceptibility to these agents as the large developmental changes are occurring. It is always good for an expectant mother to avoid teratogens, but it is especially important during the embryonic stage.
Fetal Stage (Eight to 37 Weeks)
During the fetal stage, a broad range of additional developments occur. At the beginning of the fetal stage, the eyelids close to protect the developing eyes. They remain closed until week 28. Starting about week 13, a very fine hair called lanugo begins to develop on the skin. Eventually it will cover the entire body. Prior to birth the lanugo is typically shed, but some babies will retain some of it, especially if they are born prematurely. About week 16, the fetus's genitals differentiate, such that a doctor can determine whether the fetus is male or female. The fetus seems to begin to hear about week 17, when she will respond to sounds which are loud enough to be heard inside the womb. Fingernails and toenails become apparent in week 20. Also during week 20, the mother will begin to feel the fetus squirming and kicking periodically. During week 24, the lungs develop the ability to process oxygen. While the baby is within the womb, all of the necessary oxygen is delivered from the mother's bloodstream via the umbilical cord. If the baby is born prematurely, however, the emerging lung function will be necessary for survival.
