Breathing and Exchange of Gases

Foetal haemoglobin (HbF) is a tetramer of four polypeptide chains, two alpha and two gamma chains ($\alpha_2\gamma_2$), whereas adult haemoglobin (HbA) has two alpha and two beta chains ($\alpha_2\beta_2$). So it has four subunits, not two.
The gamma chains bind 2,3-bisphosphoglycerate (BPG) less strongly than the beta chains do.
As a result, foetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, and its oxygen dissociation curve lies to the left of the adult curve.
This is essential because the foetus obtains its oxygen from the maternal blood across the placenta, where the partial pressure of oxygen is low.
The higher affinity allows foetal blood to take up oxygen released by the mother's haemoglobin.
So the haemoglobin of a human foetus has a higher affinity for oxygen than that of an adult.