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. In a normal pregnancy this usually occurs in the ninth month.
Pregnancy is divided into three-month periods, called trimesters.
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 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.