Platelet count during uncomplicated pregnancy shows considerable patient variation. To gain a better understanding of thrombocytopoiesis during pregnancy, megakaryocytes and platelets were examined during gestation and the early postpartum period, using as a model the rat. Platelet counts and megakaryocyte concentrations and DNA content distributions of timed-pregnant rats were examined at intervals from day 10 of gestation through parturition on day 22 and days 1 through 7 postpartum. Platelet survival was studied in late gestation and the early postpartum. Platelet volume was measured on gestation day 21. Platelet counts were moderately increased on gestation days 17 and 19 through 22, and on days 2 to 3 postpartum. However, the actual rate of platelet production was much higher than the platelet count suggests because the blood volume increased in late gestation to 1.5 times the nonpregnant level. Mean platelet volume and platelet volume distribution width of day 21 gestation rats were not significantly altered. Platelet survival in pregnant rats was not significantly different from that in nonpregnant females. In contrast, megakaryocyte concentration was significantly increased on gestation days 12, 17, and 19 through 21, and 2 to 3 days postpartum. In addition, in late gestation, megakaryocyte DNA content distributions displayed a marked increase in the proportion of high ploidy cells, which peaked 1 day before parturition. At that time, the proportions of 32N (43%) and 64N cells (3%) were, respectively, three and four times nonpregnant values. In contrast to megakaryocyte concentration, megakaryocyte DNA content distributions had returned to the nonpregnant pattern by day 1 postpartum. The changes in megakaryocyte DNA content distribution were accompanied by changes in megakaryocyte size. These data indicate that thrombopoiesis is substantially increased during late pregnancy, and that this increase is accomplished through an increase in megakaryocyte DNA content and size, as well as megakaryocyte number. The more rapid return of megakaryocyte DNA content than of megakaryocyte concentration to nonpregnant levels postpartum suggests that pregnancy-associated hormonal changes which produce an increase in megakaryocyte DNA content and size differ from those which cause an increase in megakaryocyte number.

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