Muter, Lynch, McCoy & Brosens — Development, 2023
Human reproduction is, by most measures, surprisingly inefficient. Only 40–60% of all conceptions survive to birth, and a large share of losses occur so early they go undetected. Yet cumulative success rates remain high — a paradox that points to a system optimized not for maximum fertility, but for quality control.
This review, published in Development, examines the biology and evolutionary context of human embryo implantation. Unlike most mammals, human implantation is interstitial: the embryo fully breaches the uterine lining and embeds in the underlying stroma, where the endometrium must transform into the decidua to sustain it.
A system built to select The authors argue that the human uterine environment — with its cyclical menstruation, spontaneous decidualization, and embryo biosensing — evolved in response to the high rate of chromosomal errors in human embryos. Rather than passively receiving an embryo, the endometrium actively evaluates it. Unhealthy embryos can disrupt the decidual reaction and trigger menstruation-like bleeding, a process the authors describe as physiological implantation failure: a protective mechanism that prevents maternal investment in a pregnancy that cannot be sustained.
The review brings together insights from evolutionary biology, single-cell genomics, and clinical reproductive medicine to reframe implantation as a dynamic, adaptable process — and one where endometrial function is just as decisive as embryo quality.