A Baby's Development Before Birth

Before birth. For a baby to develop, a sperm (sex cell from the father) must unite with an egg (sex cell from the mother). This union of a sperm and an egg is called fertilization. It produces a single cell called a fertilized egg. By a series of remarkable changes, the fertilized egg gradually develops into a baby.

A baby develops in the uterus, or womb—a hollow, muscular organ in the mother's abdomen. The period of development in the uterus lasts about nine months in most cases. During this period, development is more rapid than at any time after birth.

A fertilized egg is smaller than a grain of sand. Yet it contains a complete "blueprint" for the growth and development of a new individual. The blueprint consists of 46 tiny structures called chromosomes. Half of them come from the mother's egg, and half come from the father's sperm. Together, the chromosomes carry all the characteristics that babies inherit from their parents.

These characteristics include general body build, eye and hair color, and other physical traits as well as mental ability. To learn more about how we inherit various characteristics, see Heredity.

The developing baby. A baby goes through two major stages of development before birth. During the first stage, which lasts about two months, a developing baby is called an embryo. During the second stage, which lasts about seven months or until birth, it is called a fetus. Growth takes place during both of these stages by cell division.

The first cell division in a baby's development occurs when the fertilized egg divides in two. This division happens almost immediately after fertilization. Before the cell divides, the chromosomes duplicate themselves.

The resulting two cells thus have an identical set of 46 chromosomes each. These two cells then grow and divide, producing four cells with identical sets of chromosomes. The cells grow and divide over and over again, with each cell producing an exact duplicate of itself. The cells form an irregularly shaped, hollow ball, which becomes attached to the lining of the uterus. This tiny mass of multiplying cells forms the beginning of the embryo. A thin layer of tissue called the amniotic sac surrounds the developing embryo.

About a week after the first cell division, the cells that make up the embryo start to specialize. Groups of cells thus begin to form different organs, such as the brain and heart. The connection with the uterus develops into the placenta, an organ composed largely of blood vessels. The placenta is attached to the wall of the uterus. A tubelike structure called the umbilical cord joins the placenta to the embryo at the abdomen.

The placenta supplies everything that the embryo needs to live and grow. The placenta absorbs nourishment and oxygen from the mother's blood and transmits them to the embryo through the umbilical cord. Carbon dioxide and other waste products from the embryo travel through the umbilical cord to the placenta, which releases them into the mother's bloodstream. The placenta performs these functions until the baby is born.

A baby's development before birth. A baby begins to develop as a mass of cells in the wall of the uterus — a hollow organ in the mother's abdomen. The developing baby is surrounded by a thin inner layer of tissue called the amniotic sac and a thicker outer layer called the chorion. By the end of the second month, below left, the baby has basic human features.

Part of the chorion has become the placenta. This organ gives the baby nourishment and oxygen from the mother's blood through the umbilical cord. By the end of the sixth month, center, the baby fills the expanding uterus, and the chorion has largely disappeared. The baby is ready to be born by the end of the ninth month, right

By the end of the second month, the embryo measures about 1 inch (2.5 centimeters) long and weighs about 1/2 ounce (14 grams). Although it is small and undeveloped, it has all the basic organs and features of a human being. The developing baby is now called a fetus.

After the second month, the organs gradually mature, and the fetus shows the first signs of movement. Meanwhile, the amniotic sac has filled with a salty fluid. The fetus floats freely in this fluid, limited only by the umbilical cord. The fetus can twist and turn in every direction and even make somersaults in the amniotic sac. The mother may begin to feel these movements during the fifth month of pregnancy.

By the end of the ninth month, the fetus is well developed and ready to be born. Some infants are born prematurely — that is, before the ninth month. Premature babies born after the seventh month of pregnancy have a good chance of surviving and developing normally if they receive special medical care. Babies born as early as the sixth month of pregnancy may also survive, but complications are common.