Fetal ABG by Rex Williams:

Normal Fetal Blood Gas Values

What are the normal values for fetal blood gasses? In the fetus, the umbilical artery (UA) is traveling to the placenta. It therefore carries with it the metabolic waste products of the fetus. Hence, it has low PaO2, SpO2, and pH values, and high PaCO2 values.

Conversely, the umbilical vein (UV) is returning blood from the placenta. It therefore has higher values for PO2, SpO2, and pH, and low values for pCO2. See the table below

For simplicity in remembering the UA/UV pO2/pCO2 values, remember that UA PaO2 value is the lowest at approximately 20 torr. These values increase by 10 in a counter clockwise fashion until you reach a high value fo 50 torr in the UA pCO2 column. See table below:

One other point to note is the difference in the P50 value of fetal vs adult Hb. The P50 value is the O2 tension at which Hb is 50% saturated. Fetal Hb binds more O2 at a lower O2 tension, hence the P50 value is shifted slightly to the left to a value between 19-24 torr. An adult P50 value is approximately 26 torr. The P50 of infant approaches that of an adult at approximately 4-6 months of age reflecting the change from fetal to adult Hb.