INTRODUCTION

Despite numerous developments in assisted reproduction, the implantation rate of good-quality embryos remains low. Over the years, implantation failure has been questioned for many cases of unsuccessful in vitro fertilization (IVF) with or without intracytoplasmic sperm injection (ICSI).

Implantation is the end result of complex molecular cross-talks between the hormonally primed uterus and the blastocyst. Failure to synchronize the component processes involved in these interactions results in unsuccessful implantation. During the implantation window (day 19 to day 24 of the menstrual cycle), the endometrium undergoes precise morphological changes under the control of the sex steroid hormones – estrogen and progesterone (P). During the secretory phase, the endometrial glands display enhanced secretory activity, the endometrium becomes more vascular and edematous, and pinopodes (bulky pedunculated extrusions of the luminal epithelial cell membrane) develop on the luminal surface of the epithelium. Although these changes are useful predictors of the outcome of pregnancy, the molecular mechanisms underlying them are unknown. Of the many aspects of the synchronization process, the role of steroid hormones is indeed the best understood.

In order to improve implantation rates, a variety of therapeutic strategies have been proposed, including immunologic testing and treatment, blastocyst transfer, assisted hatching, embryo coculture, preimplantation genetic screening for aneuploidy, and embryo donation. Nevertheless, the inadequacy of luteal phase remains a cause of implantation failure in some of the patients undergoing assisted reproductive techniques (ART).