Phases of blood formation

There are three overlapping phases of blood formation during pregnancy.

Mesoblastic phase (weeks 2½ to 10)

At S6, blood formation begins in the blood islands of the wall of the umbilical vesicle. The cells inside the blood islets differentiate into megaloblasts, which are thought to be primitive nucleated erythrocytes. They contain a specific embryonic type of haemoglobin.

Hepatic phase (week 6 to birth)

Blood formation in the liver begins during week 6, overlapping with the declining mesoblastic phase. Erythropoiesis with non-nucleated erythrocytes, containing mainly fetal haemoglobin, dominates initially. From the end of month 4 onwards, megacaryocytes, granulocytes, lymphocytes and monocytes are formed. Around the same time, some blood formation (mainly lymphocytes) begins in the spleen. From month 7 onwards, blood formation in the liver decreases significantly and stops near the end of pregnancy.

Myeloid phase (month 5 onwards)

From the middle of pregnancy, haematopoietic stem cells originating from the liver invade the bone marrow. The myeloid phase of blood formation begins around this time and reaches its maximum level around birth, then decreases steadily but continues throughout life, particularly in the vertebral column, the pelvis, the sternum and the ribs. Adult haemoglobin is formed after the birth.