Endometrial receptivity and the luteal phase.

Endometrial receptivity to the implanting blastocyst determines whether pregnancy occurs. Whether fertilization is initiated in vivo or in vitro, the receptivity of the luteal-phase endometrium results from the input of interacting hormonal, growth, and immunologic factors. Endometrial adequacy for implantation has traditionally been assessed by the endometrial biopsy. Newer methods used to study the endometrium, such as ultrasound imaging and measurement of endometrial proteins, have increased our understanding yet have demonstrated the limits of our knowledge. This review is designed to analyze the available literature for concepts and scientific facts to aid in our understanding of the complex events required in preparation of a receptive endometrium. Special attention is focused on the effect of supraphysiologic levels of steroid hormones on the endometrium in ovulation induction. The effect of both embryo quality and luteal-phase support are also reviewed.