By which week is the blastocyst an embryo? A) first week
b) second week
c) forth week
d) fifth week

C) Forth week

Explanation:

In week #4 of pregnancy, the blastocyst implants in the wall of the uterus and develops into the placenta and embryo. The blastocyst is considered an embryo at the point when the amniotic sac develops (by about day 10 to 12 after fertilization, or at the start of week #5 of pregnancy).