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Fetus is a term used in biology and medicine to describe the unborn offspring of a mammal, from the time that the major structures of the body have taken shape until birth. This stage follows the embryonic stage, which is characterized by the formation of rudimentary organs and body systems.

The fetal stage is a period of rapid growth and development. In humans, this stage typically begins around the ninth week of gestation and continues until birth. During this time, the fetus grows significantly in size and weight. Organs and tissues continue to mature, becoming more complex and functional. The skeletal system ossifies, muscles develop, and the nervous system becomes more sophisticated. Sensory organs, such as eyes and ears, become more developed, and the fetus may begin to respond to external stimuli.

The duration of the fetal stage varies considerably among different mammal species, reflecting their differing reproductive strategies and gestation periods. For instance, a human fetus develops over approximately 40 weeks, while a mouse fetus develops over a much shorter period. The environment within the uterus plays a crucial role in fetal development, providing nourishment and protection.

The term "fetus" is derived from the Latin word meaning "offspring" or "young." Understanding the fetal stage is fundamental to prenatal care and the study of developmental biology.