1. Problems with the embryos
2. Problems with “host” uterus
3. Problems in the interaction between embryo and uterus.

Failure to achieve a pregnancy following 2-3 IVF cycles in which reasonably goo d [ high grade embryos] embryos were transferred is termed as implantation failure.
Embryonic loss which occurs repeatedly after Assisted Reproduction  may be attributed to many factors.
These are grouped into three categories :-

1. Decreased endometrial receptivity
2. Embryonic defect
3. Factors with combined effect