HSCs and some hematopoietic progenitors also appear by around 6 weeks of gestation in the placenta and umbilical cord blood and persist in these sites until birth.

By 7 to 8 weeks of gestation, lymphoid progenitors derived from the liver begin to seed the newly developed thymus, which is the major site of T-lymphocyte development. Hematopoiesis occurs in the spleen and lymphatic organs between 2 and 4 months.

Around 5 months of age, HSCs derived from the liver and the spleen, go to the bone marrow, which becomes the dominant source of hematopoietic elements by birth, around the time that hepatic hematopoiesis ceases.