Fetal hemoglobin (HbF) is structurally different from normal adult hemoglobin (HbA), giving HbF a higher affinity for oxygen than HbA.  HbF is composed of two alpha and two gamma chains whereas HbA is composed of two alpha and two beta chains.  The fetal dissociation curve is shifted to the left relative to the curve for the normal adult because of these structural differences:     
In adult hemoglobin, the binding of 2,3-bisphosphoglycerate (2,3-BPG) primarily occurs with the beta chains, preventing the binding of oxygen with haemoglobin. This binding is crucial for stabilizing the  deoxygenated state of hemoglobin, promoting the efficient release of oxygen to body tissues.
In fetal hemoglobin, which possesses a gamma chain instead of a beta chain, the interaction with 2,3-BPG differes because 2,3 - -BPG not binds with gamma chain as it has lower to no affinity with gamma chain.This distinction contributes to fetal hemoglobin having a higher affinity for oxygen.
Typically, fetal arterial oxygen pressures are lower than adult arterial oxygen pressures.  Hence higher affinity to bind oxygen is required at lower levels of partial pressure in the fetus to allow diffusion of oxygen across the placenta.  At the placenta, there is a higher concentration of 2,3-BPG formed, and 2,3-BPG binds readily to beta chains rather than to alpha chains. As a result, 2,3-BPG binds more strongly to adult hemoglobin, causing HbA to release more oxygen for uptake by the fetus, whose HbF is unaffected by the 2,3-BPG. HbF then delivers that bound oxygen to tissues that have even lower partial pressures where it can be released.