1. The morphologic findings of sternal bone marrow aspirates obtained from thirty healthy women during each of the three trimesters of normal pregnancy, from ten healthy women on each of the first eight puerperal days and from thirty healthy women at six weeks postpartum were compared with the findings of marrow aspirates obtained from 30 non-pregnant normal women of childbearing age.

2. Statistical analysis of the results of differential counts of these aspirates confirmed the impressions obtained from examination of the bone marrow aspirates and their differential smears.

3. Significant quantitative changes occurred in the cellular components of the bone marrow during normal pregnancy, which were maximal in the third trimester. There was a significant increase of normoblastic erythropoiesis and, to a lesser extent, of granulopoiesis during pregnancy, which was associated with an increase of immature cells (shift to the left) of both erythropoietic and of granulopoietic tissues.

4. During the first eight puerperal days the increase of erythropoiesis and of granulopoiesis diminished but did not return to normal non-pregnant normal values. At six weeks postpartum both erythropoiesis and granulopoiesis were more active than in the early puerperium and were significantly increased as compared with the findings in the marrows of normal non-pregnant women.

5. There was some increase of plasma cells and of reticulum cells in the bone marrow during pregnancy and the early puerperium.

6. No megaloblastic or macrogranulocytic changes were found in the two hundred bone marrow aspirates taken from pregnant and puerperal women or in the thirty aspirates taken from non-pregnant normal women.

7. The marrow findings of three women in the early puerperium were reported separately from the above groups. An occasional intermediate macrometamyelocyte was found in the marrows of these patients and in one a few nucleated red cells showed the earliest transitional changes toward megaloblasts. It is believed that these changes represent the earliest morphologic evidence of a deficiency of one or more of the growth factors necessary for normal hemopoiesis.

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