Prenatal development involves all of the changes that take place from fertilization until birth. Each week of a woman’s pregnancy, new milestones are achieved in development as the fetus begins to take shape.

During the first trimester the basic blueprint of human life – contained in the 46 chromosomes (23 from each partner) that determine a child’s traits – is put into action. A zygote formed after fertilization becomes a blastocyte, which then becomes an embryo composed of three layers: The ectoderm (where the brain, spinal cord and nerves, and backbone eventually develop); the mesoderm (where the heart, circulatory system, bones, muscles, kidneys and much of the reproductive system begins to develop); and the entoderm (where the lungs, intestines and bladder eventually develop).

Later during the first trimester, the embryo becomes a fetus. By the end of the first trimester, the fetus has the basics of human anatomy.

During the second trimester, the fetus’ growth rate accelerates significantly. The skin begins to form and fat stores develop. Sometime around the 19^th week of pregnancy, the mother should be able to feel the fetus’ movements. This is known as “quickening.” By the end of the 23^rd or 24^th week, some fetuses are potentially capable of surviving outside the womb, but only at very great risk.

The third trimester is the period when weight triples and length doubles as stores of protein, fat, iron and calcium increase substantially. The fetus’ movements may become constricted as space in the uterus begins to run out. Toward the end of the trimester, the fetus usually moves to a head-down position with legs tucked into the chest and knees against the nose. The fetus also will begin to move lower under the pelvis, a process known as “lightening.”