Blood Formation in the Pregnant Mouse

Pregnancy in mice was accompanied by increased erythropoiesis which was most marked in the spleen and was reflected peripherally by increased percentages of circulating erythrocytes. During the last four or five days of pregnancy, there was an equally significant drop in erythropoiesis to normal or below normal values and this occurred despite a falling hematocrit. It is suggested that the changing oxygen requirement of the fetuses may be the primary determinant of these erythropoietic changes.