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Health Library

Our Health Library information does not replace the advice of a doctor. Please be advised that this information is made available to assist our patients to learn more about their health. Our providers may not see and/or treat all topics found herein.

Embryo

When a fertilized egg has implanted in the uterus, the group of cells that will become a baby is called an embryo.

A developing, fertilized egg is known by several names within the first 2 weeks after conception, including zygote, morula (day 4), and blastocyst (day 5).

The embryonic period lasts until about 8 weeks after conception (about 10 weeks from the last menstrual period). During this time, the embryo is forming major body structures, such as the head, spine, and internal organs. This is the time when most birth defects develop. After this point, the growing baby is called a fetus.

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