Nick Macklon examines endometrial receptivity — and proposes changing the lens: the endometrium is not passive "soil" but an active partner of the embryo.
The talk's central metaphor is "the endometrium as the embryo's partner." Implantation is a dialogue between two sides, not a one-way "taking root" of the embryo in inert tissue.
A key role is given to stromal cells: they can recognise embryo quality and respond differently to a competent and an incompetent blastocyst.
Hence the concept of a biosensor function: a healthy endometrium can reject non-viable embryos, and a breakdown of this mechanism leads either to repeated losses or to repeated implantation failures.
Macklon stresses that the implantation window is individual: in different patients it is shifted in time, and the "average" timing does not suit everyone.
This explains some failures when transferring good-quality euploid embryos: the problem is not the embryo but desynchronisation with the individual receptivity window.
An important thesis — receptivity lies beyond embryo genetics. Even a chromosomally perfect blastocyst does not implant if the endometrium is not ready or rejects it.
This shifts the diagnostic emphasis: besides assessing the embryo, one must assess the functional state of the endometrium, not just its thickness on ultrasound.
Macklon is cautious about "blind" add-ons and commercial tests, stressing that understanding the mechanism matters more than mechanically applying panels.
The aim of these directions is to move from a static "picture" of the endometrium to a dynamic assessment of its readiness for dialogue with the embryo.
The practical sense for the clinic is to think of the "embryo + endometrium" pair as a system, rather than optimising only the embryological side while leaving the endometrium a "black box."
Macklon also draws practical consequences: if the endometrium actively assesses the embryo, then "sorting" happens not only in the lab but in the uterus, and some early losses are not a failure but the biosensor at work, rejecting embryos that were never viable.
Hence caution with attempts to "force" an unsuitable endometrium to accept an embryo through immunosuppression: by suppressing the protective mechanism one may obtain implantation where it was doomed from the start, at the cost of late losses.
The upshot: receptivity is an active, individual and still largely unexplored process; the future lies in functional endometrial markers, while today's practice already benefits from seeing the endometrium as a partner, not a backdrop.
Receptive or non-receptive? A continuum-based view of the endometrium as an active biosensor that selects against poor-quality embryos. Why the classical 'all in the egg' narrative misses recurrent implantation failure — and how a new diagnostic toolbox is taking shape.
Professor and Medical Director of the London Women's Clinic and Co-Chief Editor of Reproductive BioMedicine Online. Internationally recognized for his research on ovarian stimulation, endometrial receptivity and periconception. Author of more than 200 scientific papers.
Author of the lecture "Endometrial receptivity: from biosensor to clinical practice"
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Audio: UASubtitles: UA32 min
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