WebMD
Font Size

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.

ByHealthwise Staff
Primary Medical ReviewerSarah Marshall, MD - Family Medicine
Specialist Medical ReviewerKirtly Jones, MD - Obstetrics and Gynecology
Last RevisedApril 26, 2013

WebMD Medical Reference from Healthwise

Last Updated: April 26, 2013
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.