Erythropoiesis is established soon after implantation of the blastocyst, with primitive erythroid cells appearing in yolk sac blood islands by day 18 of gestation. Ie the first blood island appear in mesoderm at around 3 weeks of development.
How does yolk sac erythroblasts differ from their later definitive counterparts ?
 Primitive erythroblasts differentiate within the vascular network rather than in the extravascular space and remain nucleated as they circulate.
 Primitive erythroblasts are characterized by more rapid maturation, increased sensitivity to erythropoietin, and a shortened life span compared to fetal and adult erythroblasts.
 Yolk sac erythroblasts are extremely large red cells c/a megaloblasts
After 7 weeks gestation, hematopoietic progenitors are no longer detected in the yolk sac but yolk sac derived primitive erythroblasts continue to circulate until approximately 12 weeks of gestation.
SO HEMATOPOIESIS IN YOLK SAC = 3rd to 7th WEEK
Note that blood islands in yolk sac have HEMANGIOBLASTS arising from MESODERM. Hemangioblasts in centre of island form bood cells and are callled as HEMATOPOIETIC STEM CELLS and those in perifery differentiate into ANGIOBLASTS which form blood vessels.
Blood cells arising in blood islands of yolk sac is a transitory phenomena as the definitive hematopoietic cells arise from mesoderm surrounding aorta called as AORTA-GONAD-MESONEPHROS REGION (AGM Region). These cells will first colonise the liver which becomes major hematopoietic organ of liver and later on these stem cells from liver will colonise the bone marrow.
BFU-E appear in the fetal liver as early as 5 weeks of gestation, and CFU-E are evident soon thereafter. The liver serves as the primary source of red cells from the 9th to the 24th weeks of gestation.
In contrast to the yolk sac, where hematopoiesis is restricted to erythroid and macrophage cells, hematopoiesis in the fetal liver also includes other myeloid as well as lymphoid lineages. So some “may be asked- NOT so important points” ( DNB oneliners)
 First RBC’S = 3rd week( yolk sac)
 Megakaryocytes are present in the liver by 6 weeks of gestation and Platelets are first evident in the circulation at 8–9 weeks gestation.
IMPORTANT NOTE POINT : The liver remains the primary site of erythropoietin transcription throughout fetal life as compared to kidney in adults.
Hematopoietic cells are first seen in the marrow of the 10- to 11- week embryo and they remain confined to the diaphyseal regions of long bones until 15 weeks gestation.The marrow becomes the major site of hematopoiesis after the 24th week of gestation.
NOTE : Lymphopoiesis is present in the lymph plexuses and the thymus beginning at 9 weeks gestation.
 Hb PORTLAND = Zeta & gamma chains**
 Hb GOWER 1 = Zeta & epsilon chains **
 Hb GOWER 2 = Alpha & Epsilon chains**
FETAL Hb = HbF = Alpha & Gamma chains**
Hb Gower 1 is the major hemoglobin in embryos less than 5 weeks of gestation****Q
Hb Gower 2 has been found in embryos with a gestational age 4-13 weeks.
Hb Portland is found in young embryos but persists in infants with homozygous thalassemia.
At 10 -11 weeks fetal Hb becomes predominant Hb****Q
Adult Hb appears as early as 16- 20 weeks but synthesised in a nearly exclusive manner after 38 weeks.
Ref :  Langman’s medical embryology/10ed /pg 77