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Pregnancy is the nine-month process in which a baby develops inside a woman's womb or uterus. This process is called fertilization and the resulting cell is called a zygote. The zygote undergoes cell division and keeps dividing as it passes through the Fallopian tube and is implanted in the uterus. This occurs in about one week's time and the implanted ball of eight cells is then called a blastocyst.

With the blastocyst implanted in the endometrium of the uterine wall, the woman may be said to have conceived and to be pregnant. The endometrium provides the fertilized egg with a natural nesting place and immediate nutrition.

Also at this time, the placenta — a very important exchange and filtering system — begins to develop between mother and baby. Oxygen and nourishment from the mother's blood are filtered through the placenta to the baby, and waste products from the child are returned to the mother through the placenta. The baby and the placenta are connected by the umbilical cord.

A few days after conception, a transparent sheath called the amniotic sac begins to grow around the baby. The amniotic sac fills with special fluid, which acts as a cushion to keep the developing baby safe from outside bounces and shocks. Each day the fluid in the sac is exchanged for new fluid in a continual replacement system.

Typically, just before the birth of the baby the sac breaks, releasing about a quart of water through the vagina. When the amniotic sac releases the fluid, it is said the woman's "water broke" and the birth of the baby, its passage from inside the mother out into the world, is imminent. In a normal pregnancy this usually occurs in the ninth month.

Pregnancy is divided into three-month periods, called trimesters. The first trimester is the first, second, and third months of pregnancy; the second trimester is the fourth, fifth and sixth months; and the third and last trimester is the seventh, eight, and ninth months.

Pregnancy — The First Trimester

Despite the woman's unchanged external appearance during the first trimester of pregnancy, many important developments are taking place within her body. By the end of the first month, the developing embryo is about one-tenth of an inch long, has a beating heart, has the beginnings of a head, spinal cord, nervous system, lungs and the buds of arms and legs.

During this phase of critical development the embryo is particularly sensitive to influences which could cross the placenta, such as drugs (including alcohol) or certain infections.

During the second and third months of the first trimester the embryo continues to develop such features as bone cells, eyes, ears, nose, fingers, feet and toes. The refinement of body parts also includes teeth sockets, and the beginning of fingernails.

The budding of the clitoris, and the budding of the penis and scrotum are also taking place during the second month, but the sexual organs are not refined enough to distinguish as male or female until sometime in the third month. The extraordinary process of creation continues day after day, yet the fetus, as it is called at eight weeks, is still only two to four inches long and weighs less than one ounce. But it is already looking unmistakably human.

Pregnancy — The Second Trimester

During the second trimester the major body systems and organs are still being refined. Facial features are molded, eyebrows and eyelashes begin to appear and the eyelids can open and close. Facial expressions, such as frowning, lips that open and close and turning of the head begin to appear, but it is not clear that these gestures can be interpreted to mean anything.

The skin of the fetus is very thin and transparent, clearly showing the blood vessels lying just below. Roughly halfway through pregnancy, muscles have developed enough to allow the fetus to move its arms and legs. This is the time when a woman begins to notice the first fluttery fetal movements, a stage termed "quickening".

During the second trimester, the heartbeat of the fetus can be heard with a stethoscope and the fetus will grow rapidly, reaching approximately 2 pounds in weight and 14 inches in length.

Pregnancy — The Third Trimester

During the third trimester it is typical for the fetus to toss and kick quite a bit, making its presence obvious to the mother. These movements are a sign that the nerve fibers of the fetus are developing properly. They are necessary for muscular and skeletal growth, and for the development of fine motor ability.

In the seventh month and the first part of the eighth the fetus gains weight and grows tremendously. It generally triples in weight and increases in length by 5 or 6 inches. The expectant mother's abdomen becomes exceedingly large toward the end of pregnancy and may cause some back pain and frequent urination due to the fetus applying pressure to the mother's bladder.

In the eighth and ninth months the baby's organs and structures are developed enough to function on their own. During the final month of pregnancy the baby, who has been in an upright position, gradually turns completely over until its head is pointing downward. It is then ready to be born, as soon as contractions of the uterus begin to push the baby out through the vaginal canal and into the waiting world.