Development happens quickly during the prenatal period, which is the time between conception and birth. This period is generally divided into three stages: the germinal stage, the embryonic stage, and the fetal stage.Stage 1: The Germinal Stage
The two-week period after conception is called the germinal stage. Conception occurs when a sperm cell combines with an egg cell to form a zygote. About thirty-six hours after conception, the zygote begins to divide quickly. The resulting ball of cells moves along the mother’s fallopian tube to the uterus.
Around seven days after conception, the ball of cells starts to become embedded in the wall of the uterus. This process is called implantation and takes about a week to complete. If implantation fails, as is quite common, the pregnancy terminates. One key feature of the germinal stage is the formation of a tissue called the placenta. The placenta has two important functions:
The embryonic stage lasts from the end of the germinal stage to two months after conception. The developing ball of cells is now called an embryo. In this stage, all the major organs form, and the embryo becomes very fragile. The biggest dangers are teratogens, which are agents such as viruses, drugs, or radiation that can cause deformities in an embryo or fetus. At the end of the embryonic period, the embryo is only about an inch long.Stage 3: The Fetal Stage
The last stage of prenatal development is the fetal stage, which lasts from two months after conception until birth. About one month into this stage, the sex organs of the fetus begin to form. The fetus quickly grows as bones and muscles form, and it begins to move inside the uterus. Organ systems develop further and start to function. During the last three months, the brain increases rapidly in size, an insulating layer of fat forms under the skin, and the respiratory and digestive systems start to work independently.
Although the womb provides protection, the fetus remains indirectly connected to the outside world through its mother. Several factors that are linked to the mother can harm the fetus: