When does implantation occur occurs during the

 When does implantation occur?:
 What occurs during the embryo/embryonic period?:
 What are the 3 cell layers the embryo has during the embryonic period?:

 When is and what occurs during the fetus/fetal period?:
 How do these three periods compare to trimesters?:
 Define placenta, umbilical cord, HCG, amnion:
 How far in gestation is the age of viability?: