Explanation:
The oxyhaemoglobin dissociation curve represents the relationship between the partial pressure of oxygen (pO2) and the percentage of haemoglobin that is saturated with oxygen (% saturation). The shape of the curve indicates how readily haemoglobin binds and releases oxygen.

Maternal Haemoglobin:
The oxyhaemoglobin dissociation curve of maternal haemoglobin is a typical sigmoid curve.
- At low pO2 values (in tissues), maternal haemoglobin has a higher affinity for oxygen, which means it binds oxygen strongly.
- As pO2 increases (in lungs), maternal haemoglobin releases oxygen more readily.

Foetal Haemoglobin:
The foetal haemoglobin has a higher affinity for oxygen compared to maternal haemoglobin. This is essential for the transfer of oxygen from the maternal circulation to the foetal circulation across the placenta.
- The foetal haemoglobin dissociation curve is shifted to the left compared to the maternal curve.
- This means that at any given pO2 value, foetal haemoglobin has a higher % saturation compared to maternal haemoglobin.
- The left shift indicates that foetal haemoglobin binds oxygen more strongly and releases it less readily than maternal haemoglobin.

Reason for Left Shift:
The left shift in the foetal haemoglobin dissociation curve can be attributed to the following factors:
1. Decreased affinity for 2,3-BPG: Foetal haemoglobin has a lower affinity for 2,3-bisphosphoglycerate (2,3-BPG) compared to adult haemoglobin. 2,3-BPG is a molecule that binds to haemoglobin and decreases its affinity for oxygen. Thus, the lower levels of 2,3-BPG in foetal haemoglobin enhance its oxygen-binding capacity.
2. Different structure: Foetal haemoglobin has a different structure compared to adult haemoglobin. It has two gamma chains instead of two beta chains found in adult haemoglobin. This structural difference increases the affinity of foetal haemoglobin for oxygen.
3. Higher pH: The foetal blood has a higher pH compared to maternal blood due to the production of alkaline compounds by the placenta. The higher pH further increases the affinity of foetal haemoglobin for oxygen.

Conclusion:
In conclusion, the foetal haemoglobin dissociation curve is shifted to the left and is sigmoid in shape. This left shift and higher affinity for oxygen allow foetal haemoglobin to efficiently extract oxygen from maternal blood in the placenta, ensuring adequate oxygenation for the developing foetus.

As you know that efficiency of foetal RBC will be more then adults. So even at low pO2 (as compared to adults) %age of saturation will be high. so graph will move to left and sigmoid