Due to the correlation between the amount of hemoglobin F and F-cells, F-cell numbers are higher in some inherited hemoglobin disorders, including beta-thalassemia, sickle cell anemia and hereditary persistence of fetal hemoglobin. Additionally, some acquired conditions can also have higher F-cell numbers, such as acute erythropoietic stress (response to poor oxygenation which includes very rapid synthesis of new red blood cells  and pregnancy. F-cells have similar mass of haemoglobin per cell compared to red blood cells without haemoglobin F, which is measured mean cell haemoglobin values (MCH). There is a significant increase in hemoglobin F levels during early pregnancy. However, it's not clear whether these levels are stable or decrease as the pregnancy goes on, as different sources reported different results. The increase in hemoglobin F then induces a 3 to 7 fold increase in the number of F-cells in pregnant women, which was observed between the 23rd to 31st week of gestation. However, as to the reason of the increase in hemoglobin F levels in pregnant women, there doesn't seem to be conclusive evidence. While an early study suggested that maternal red blood cells switch on hemoglobin F production during pregnancy, more recent literature suggested that the increase in haemoglobin F could be, at least in part, due to fetal red blood cells being transferred to the maternal circulation. Presence of high levels of hemoglobin F in pregnant women can impact the growth of the fetus, as fetal red blood cells struggle to compete for the oxygen from the mother's circulation. This is because instead of competing with hemoglobin A, which has a weaker association to oxygen than hemoglobin F, it becomes a competition between fetal and maternal hemoglobin F which have similar affinities for oxygen. As a result, women with hemoglobin F as >70% of total hemoglobin are much more likely to have fetuses that are small for their gestational age compared women with <70% hemoglobin F (at a rate of 100% compared to 8%, respectively). This is a rare benign genetic disease where production of hemoglobin F persists after twelve months of life and into the adulthood. As a result, hemoglobin F is present in a higher number of adult red blood cells than normal. It doesn't present symptoms and is usually discovered when screening for other blood-related diseases. In this condition, the genes coding for the γ subunit (HBG1 and HBG2) are not suppressed shortly before birth.