When does switchover from fetal to adult hemoglobin synthesis begin –
Fetal Hb → (α2 γ2)
Adults Hb → (α2 β2)

During the initial fetal life, the major haemoglobin synthesized in the body is fetal haemoglobin, because, in the fetus, the chain synthesized predominantly is the γ chain.
the β chain is present in only trace amount in early embryos
The rate of synthesis of y and β chains and the amount of HbA and HbF are inversely related during the intrauterine stage.
As the age of the fetus increases, the synthesis of β chains and the amount of adult haemoglobin in the blood increases
At 36 weeks of gestational age the rate of synthesis of (β chain exceeds that of γ chain. This is called switch over.
14 weeks gestation
30 weeks gestation
36 weeks gestation
7–10 days postnatal