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Home > Health Library > 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.
Current as of: June 16, 2021
Author: Healthwise Staff
Medical Review:Sarah Marshall MD - Family Medicine & Adam Husney MD - Family Medicine & Kirtly Jones MD - Obstetrics and Gynecology
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