Over the course of approximately 38 weeks, a fertilized egg grows from zygote to embryo to fetus. The development that takes place before birth--referred to as prenatal development--follows a set of steps that are very consistent in order and timing. Medical researchers have described prenatal development in tremendous detail.
Post-Fertilization vs. Gestational Age
Two different age metrics are used to describe prenatal development. The first is the post-fertilization age--the time that has passed since fertilization of the egg. The second is the gestational age--the time elapsed since the first day of the mother's last menstrual cycle before pregnancy. Gestational age is approximately two weeks greater than post-fertilization age. Gestational age is often used because its start date can be clearly determined, whereas the moment of fertilization must be inferred.
Doctors often alternate between these two age description systems, which can lead to confusion. This article will refer to the developmental stages in terms of the post-fertilization age.
Zygote Stage (0 to 5 days)
Zygote is the medical term for a fertilized egg. Fertilization occurs when a sperm penetrates an egg, usually in one of the Fallopian tubes. Half of the father's DNA combines with half of the mother's DNA to form a single, complete set of human genetic information. If two eggs are released in the same menstrual cycle, two zygotes may be formed that ultimately become fraternal twins. (Identical twins form if the embryo splits to form two separate bodies during that stage of development.) Regardless, upon forming, a zygote immediately begins to divide and grow into a ball of cells called a blastocyst.
Blastocyst Stage (5 to 10 days)
About one week after fertilization, a ball of several hundred cells implants itself in the lining of the mother's uterus. The blastocyst is only one layer of cells thick in all but one section, where it's about four cells thick. The thin layer becomes the placenta, an organ responsible for regulating development that serves as an intermediary between the body of the mother and the embryo. The thick section of the blastocyst will become the embryo itself.
Part of the blastocyst forms a thin membrane called the amniotic sack that fills with clear fluid. When a section of the blastocyst begins to float in this amniotic fluid, the embryo stage has begun.
Embryonic Stage (2 to 7 weeks)
During the embryonic stage, the major internal organs and external body structures take on identifiable shapes. The heart forms and begins to pump blood. The brain and spinal cord can be identified. Arms and legs sprout and take shape. By the end of this stage, fingers and toes are discernible. Hair begins to appear. Near the end of the embryonic stage, recordable brain activity appears and influences the movements of muscles.
This is the stage of prenatal development during which drugs, toxins, radiation and infection can be most damaging. Expectant mothers should take care to avoid these as much as possible.
Fetal Stage (8 to 37 weeks)
By the beginning of this stage, the fetus has all of the body parts that an adult has. The muscles cause the body parts to move in coordinated ways, and the organ systems begin to function. For example, the stomach begins to produce digestive juices. The fetus gets all of his oxygen from the mother via the placenta, but breathing of the amniotic fluid occurs. Similarly, the fetus swallows and expels amniotic fluid from the digestive system.
By 14 weeks old, the sex of the fetus can be identified in an ultrasound scan. Between 16 and 20 weeks, the mother can typically start to feel the baby move. This is especially true as the fetus becomes larger. In this stage, the fetus grows from less than 1 oz. to more than 6 lbs. By 24 weeks, the fetus has a chance of surviving outside of the womb if a premature birth occurs.
The three stages of prenatal development described here--blastocyst, embryo and fetus--are based on anatomy and function. Others describe fetal development and pregnancy in terms of three trimesters, which are based on approximately three-month stages of the pregnancy. The first trimester is from Weeks 1 to 12, the second from Weeks 13 to 27, and the third from Weeks 28 to 42. Note that trimesters are referred to in terms of gestational age, rather than post-fertilization age.