Abstract
This study shows a minor, statistically non significant increase, of β-chain synthesis from 18 to 24 weeks gestation in both normal and β°-thalassemia heterozygous fetuses with no significant changes in the ratio between the values for the two groups as a function of gestational age. This result indicates that at midtrimester pregnancy the developmental pattern of Hb F to Hb A switching in B O-thalassemia heterozygotes is similar to that of normal fetuses.