The gestational period is 38 weeks from fertilization to birth. The developing organism is called an embryo up to week 8 and a fetus after week 8.
Once the ovum is fertilized, the zygote begins to divide. Cleavage is the division of cells in the early embryo. The result is a morula, a solid ball of cells within the zona pellucida (8 cells, 3-4 days post-fertilization). The morula continues to divide, becoming a blastocyst, with two cell layers and a cavity (4-5 days post-fertilization). The embryoblast (inner cell mass) of the blastocyst becomes the embryo. The trophoblast is its outer layer of cells, which becomes the placenta and other extra-embryonic tissues. The blastocoele is its fluid-filled cavity.
On implantation, the blastocyst becomes completely buried in the endometrium vagina (10 days after fertilization). At this point, the placenta begins to develop. Gastrulation is the infolding of the dorsal layer of the embryonic disc to form three germ layers (day 14): endoderm, mesoderm, and ectoderm. The endoderm forms the future gut and epithelial linings of organs. The mesoderm forms the muscles, dermis, bone, cartilage, and blood cells. The ectoderm forms the nervous system, epidermis, and enamel of the teeth. By week 4, the major body systems have begun to develop with minimal independent function.
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