The timing of implantation

There is evidence that the uterus is receptive to an implanting blastocyst from approximately 7 to 11 days after the luteinising hormone (LH) peak. Effective receptivity may well end in the human when corpus luteum rescue is no longer feasible,rather than as a result of a change in uterine properties. Thus the risk of early pregnancy loss increases with later implantation, from 13% on day 9 after ovulation to 82% on day 12 or later. Many conceptuses fail to implant because of genetic or metabolic abnormalities. Poor uterine receptivity has been widely proposed as another cause of implantation failure, but it remains unclear to what extent this may be because of defective synchronisation between embryo development and maternal differentiation, undetected embryo abnormalities, or a real failure of uterine capability to support implantation of a normal conceptus. For practical reasons, the term ‘implantation failure’ is often used to include a range of outcomes from blastocyst non-attachment to biochemical (human chorionic gonadotrophin (hCG)-positive) pregnancies that do not give rise to a functioning embryonic heart, that is to say, those that fail in the first 4—5 weeks. Such a definition encompasses implantation and placental organogenesis; this article mostly addresses the former, drawing pertinent comparative information from animal models. Knowledge of early placental development is emerging from mouse genetics and culture models but this lies outside the scope of the present discussion.