How is an embryo different from a foetus?

An embryo represents the early developmental stage immediately following fertilization, characterized by rapid cell division, differentiation, and formation of basic body structures and organ systems. In humans, the embryonic period extends from conception through approximately the eighth week of pregnancy. During this critical phase, the single-celled zygote undergoes cleavage to become a ball of cells, which implants in the uterine wall and develops into recognizable structures including the neural tube (future brain and spinal cord), primitive heart, and rudimentary limb buds. The embryo is extremely vulnerable to environmental influences during this period, as fundamental body architecture is being established.

A foetus is the term applied to the developing organism after the embryonic period concludes and continuing until birth. In humans, the fetal stage begins around the ninth week when all major organ systems have formed, though they remain immature and require further development and growth. The fetal period focuses primarily on growth, maturation, and refinement of structures rather than their initial formation. For instance, a nine-week fetus possesses recognizable human features—formed limbs with fingers and toes, facial features, functioning organs—but weighs only about 30 grams. Over the remaining months of pregnancy, the fetus grows from this small size to a birth weight averaging 3-4 kilograms, with organs developing the functional capacity needed for independent survival. This distinction between embryo and fetus is clinically significant, as developmental abnormalities occurring during the embryonic stage typically affect basic organ formation, while fetal-stage issues more often involve growth restriction or functional maturation problems.