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:
- Passing oxygen and nutrients from the mother’s blood into the embryo
or fetus
- Removing waste materials from the embryo or fetus
Stage 2: The Embryonic Stage
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.
Adverse Factors Affecting Fetal Development
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:
- Poor nutrition
- Use of alcohol
- Smoking
- Use of certain prescription or over-the-counter drugs
- Use of recreational drugs such as cocaine, sedatives, and narcotics
- X-rays and other kinds of radiation
- Ingested toxins, such as lead
- Illnesses such as AIDS, German measles, syphilis, cholera, smallpox,
mumps, or severe flu
