0-40 hours: The fertilized ovum is called the zygote and consists of two to four cells. It rests in the fallopian tube.

40-72 hours: Having grown to 12 to 16 cells, the morula floats into the uterus and grows to 64 cells.

4-8 days: The blastocyst continues to grow to more than 100 cells. The inner cell mass becomes the human organism itself. The trophoblast forms between the inner mass and the environment; it will later develop into the placenta. By day 6 or 7, the blastocyst implants on the uterine wall, i.e., endometrium, in a process called nidation.

8-13 days: Cells separate and arrange themselves into three embryonic germ layers, which give rise to all of the major organs. The ectoderm—the outer layer—gives rise to the nervous system (including the brain and spine), skin, nails, hair, and salivary, pituitary, and mammary glands. The endoderm—the innermost layer—gives rise to the thyroid, bladder, lungs, and digestive system. The mesoderm—which emerges between the ectoderm and the endoderm—gives rise to the heart, circulatory and lymph systems, connective tissue, muscle, and bones. By the end of the 2nd week, the placenta, umbilical cord, and amniotic sac have taken shape.

Third week: An indentation of cells, called the neural plate, forms in the ectoderm, giving rise to the brain (hindbrain, midbrain, forebrain) and neural tube (spinal cord). The chambers of the heart and blood vessels arise from a similar process in the mesoderm.

Fourth week: The heart begins to beat. Limb buds are visible. Eyes, ears, nerves, and the skeletomuscular and digestive systems begin to form. Vertebrae are present; major veins and arteries are completed. The neural tube closes. The embryo is approximately 0.5 cm long and weighs 0.4 g.