Why is an endometrial biopsy performed?

An endometrial biopsy is done in some women with fertility problems to correlate the endometrium with the cycle day and to see whether there could be problems with the implantation of a fertilized egg. If the correlation is good and the endometrial biopsy is IN PHASE, it is confirmed that the lining of the uterus is receptive to an implanting pregnancy. An OUT OF PHASE endometrium, an endometrium that's several days behind of what it is supposed to be, may imply that there is a luteal phase defect.