Answer
A developing fetus get oxygen from the maternal Haemoglobin in womb.
Work Step by Step
For adults during respiration aerial oxygen present in our lung diffuses into capillary and when it get in contact with blood form acompound with haemoglobin according to the following chemical equation.
$Hb + O_{2} ⇌ HbO_{2}$
When blood flows through lung the above equilibrium shift toward right side (product) due to higher concentration of oxygen in lung.
On the other hand when blood flows out of lung the above equilibrium shift toward left side (reactant) due to lower concentration of oxygen in muscle and organ.
This happens according to Le Chatelier’s principle
Similar equilibrium reaction occurs in case of fetus and this is as follows-
$HbF + O_{2} ⇌ HbFO_{2}$
The value of equilibrium constant are different for these two above reactions.Due to higher value of equilibrium constant of the reaction occurring with fetal haemoglobin,it can combine with oxygen at low concentration of oxygen . In placenta maternal blood and fetal blood flows in close proximity.Oxygen released by maternal haemoglobin binds with fetal haemoglobin and carries into its circulatory system.
