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Hi. I do research pertaining to this disease and can answer your very thoughtful question. It is in fact safe for pregnant women to have high levels of fetal hemoglobin. There is a condition known as Hereditary Persistence of Fetal Hemoglobin (HPFH) in rare individuals who express near 100% fetal hemoglobin into adulthood, and they are able to undergo pregnancy perfectly normally with no harm to mother or child. While it is true that HbF has a mildly higher affinity to oxygen, it has not been shown in population-based studies that having higher levels of HbF by itself causes any actual clinical adverse events.