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James A. O’Brien

The Difference Between an Embryo and a Fetus

Pregnancy jargon explained.

The difference between an embryo and a fetus has to do with the different phases of baby’s development. “The embryo is defined as the developing pregnancy from the time of fertilization until the end of the eighth week of gestation, when it becomes known as a fetus,” says James A. O’Brien, MD, medical director of inpatient obstetrics at Women & Infants Hospital of Rhode Island.

During the embryonic period, cells begin to take on different functions. The brain, heart, lungs, internal organs and arms and legs begin to form. Once baby is a fetus, growth and development are aimed at prepping baby for life on the outside.

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