Thrombocytopenia is an initial maternal response to fertilization in mice.

There was an increase in weight of the spleens of pregnant and pseudopregnant mice in the first week after mating, but the increase occurred on Day 4 in pregnant mice and Day 2 in pseudopregnant mice. The retardation of the presumed hormonally induced increase in spleen weight during pregnancy corresponded with a significant reduction in the splenic platelet pool. This response by the spleen to early pregnancy suggested that platelets were being supplied to the vascular pool. There was a significant reduction in the platelet count by 10:30 h on the day of mating in pregnant mice and persisted until Day 7 of pregnancy, then returning to normal levels. This response did not occur in pseudopregnant mice. The decrease in platelet count was dependent upon the presence of fertilized eggs. It did not occur in mice sterilized by bilateral ligations of the oviducts and mated with fertile males. Thrombocytopenia did occur within 3 h of transfer of fertilized eggs to pseudopregnant recipients and the magnitude of the response was significantly correlated (b = -0.86) with the number of embryos present in the reproductive tract. An initial systemic response to pregnancy in mice was therefore an increased vascular demand for blood platelets, resulting in a significant reduction in the splenic and peripheral blood platelet concentration.