Skip to Content
Home > Wellness > 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:
May 22, 2015
Sarah Marshall, MD - Family Medicine & Kirtly Jones, MD - Obstetrics and Gynecology
To learn more about Healthwise, visit Healthwise.org.
© 1995-2015 Healthwise, Incorporated. Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated.
Our interactive Decision Points guide you through making key health decisions by combining medical information with your personal information.
You'll find Decision Points to help you answer questions about:
Get started learning more about your health!
Our Interactive Tools can help you make smart decisions for a healthier life. You'll find personal calculators and tools for health and fitness, lifestyle checkups, and pregnancy.
Feeling under the weather?
Use our interactive symptom checker to evaluate your symptoms and determine appropriate action or treatment.