Early pregnancy loss: A critical review of the evidence for embryo “wastage”

Abstract

Approximately 10% of clinically recognized pregnancies end in miscarriage. Many authors suggest that this number greatly underestimates the death of early human embryos, commonly asserting that in addition to clinical pregnancy loss, as many as 60% of all conceptions are lost before a recognized pregnancy is established. However, a critical review of the literature indicates that previous analyses of early embryo loss have relied on inadequate data, unwarranted speculation and insufficient clarity about what constitutes a human embryo. Anovulation, fertilization failure and non-development (none of which entail embryo demise) make significant contributions to observed fecundity, yet have often been ignored. Although the actual percentage of early human embryo loss is not known with precision, based on multiple lines of evidence, preimplantation embryo loss is likely to be negligible (if it occurs at all). When all early pregnancy loss is considered, embryo wastage does not exceed 35%, and is likely to be lower than this maximum figure, possibly as low as 7%. In light of this conclusion, the relevance of embryo wastage to an assessment of the moral worth of the human embryo is briefly considered.