How is an embryo defined?

An embryo is defined as an unborn infant during the first three months of development, which corresponds to the stages of prenatal development from fertilization to the end of the 12th week, or roughly the first trimester. During this time, the basic structures and organ systems of the body begin to form. This period is crucial for the development of the embryo, as it undergoes significant cellular differentiation and growth processes that lay the foundations for all subsequent development.

The other choices do not accurately represent the stage of development referred to as an embryo. A newborn infant typically refers to a child who has already been delivered and is outside the womb, while a child in school pertains to individuals who are beyond the embryonic and fetal stages and are usually at least several years old. A type of medical professional does not relate to developmental stages and is entirely different from the biological processes involved in embryology. Hence, the definition centered on the first three months aligns correctly with the scientific understanding of an embryo.