Abstract
BACKGROUND: When a competent blastocyst stage embryo finds itself in an unreceptive uterus, it delays development. In around one hundred species representing various orders, this delay is known to be reversible, but this phenomenon - termed embryonic diapause (ED) - is not considered a general characteristic of all mammals. PRESENTATION OF THE HYPOTHESIS: Recently, however, we demonstrated that a non-diapausing species, the sheep, is capable of ED, suggesting the hypothesis that this is in fact an ancestral trait common to all mammals, including humans. TESTING THE HYPOTHESIS: In spite of the obvious difficulties in testing this idea, we propose a combination of indirect observations on human fertility patients, and direct study of the embryos of non-human primates. IMPLICATIONS OF THE HYPOTHESIS: Support for our hypothesis would require revision of obstetric interventions routinely performed when a human pregnancy extends beyond the due date.