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.