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.