Rethinking the window of implantation: Evidence from extrauterine pregnancies supports a two-stage model of human implantation

Background The window of implantation (WOI) paradigm holds that successful human embryo implantation requires a narrow, hormonally regulated period of endometrial receptivity. Yet extrauterine pregnancies occur in 1–2% of conceptions, establishing in anatomical sites entirely devoid of endometrial tissue or temporal hormonal preparation, a fundamental contradiction with WOI exclusivity that warrants critical reexamination. Objective To evaluate whether evidence from extrauterine pregnancies supports or refutes the concept of an exclusive endometrial window of implantation. Design Critical review of peer-reviewed literature published between January 1980 and February 2026. Main Findings Extrauterine implantation occurs across remarkably diverse sites, including the fallopian tube (93–97% of ectopic cases), ovary, omentum, peritoneum, abdominal organs, and even endometrial carcinoma. Pregnancy progression has been documented to advanced gestation, including ovarian pregnancies at 34 weeks and abdominal pregnancies that delivered live neonates at 41 weeks. These pregnancies are established through embryo-driven mechanisms: autonomous trophoblastic invasion via matrix metalloproteinases, local immune modulation via IL-10 and TGF-β1 secretion, and compensatory angiogenesis via upregulation of VEGF and FGF2. A consistent pattern unites these otherwise disparate tissues, namely a rich vascular supply combined with systemic mid-luteal progesterone exposure from a functioning corpus luteum at the moment of attachment. We propose that these two features, rather than tissue-specific endometrial preparation, constitute the minimal sufficient conditions for trophoblastic attachment, while the decidualized endometrium remains uniquely qualified to sustain pregnancy to term. Conclusion Extrauterine pregnancies constitute natural experiments demonstrating that human implantation is more flexible and embryo-driven than the WOI paradigm allows. The endometrial window represents optimal rather than essential conditions for attachment, with profound implications for assisted reproductive technology, particularly the questionable utility of endometrial receptivity testing and the substantial overdiagnosis of recurrent implantation failure.