Bleeding from the genital tract is an invariable concomitant of reproduction in many species of vertebrates. Most familiar is the macroscopic blood loss that occurs after parturition in mammals with deciduate placentas. Less well known is the microscopic bleeding of maternal origin that accompanies the early stages of placentation and embryonic development. It is the purpose of this paper to summarize the evidence for this phenomenon based on observations in other species and to present evidence for its occurrence in man.
BLEEDING IN LOWER VERTEBRATES
Throughout the vertebrate subphylum, whenever the embryo is dependent on maternal sustenance beyond the ovum stage, red blood cells may be found in admixture with the pabulum of the brood chamber. In a delightful little essay on the phylogeny of menstruation, Carl Hartman1 called attention almost a quarter of a century ago to the purposeful nature of this maternal bleeding, even as it occurs
Speert H, Guttmacher AF. FREQUENCY AND SIGNIFICANCE OF BLEEDING IN EARLY PREGNANCY. JAMA. 1954;155(8):712-715. doi:10.1001/jama.1954.03690260004002