A derivative of haemoglobin, also known as carbon monoxide hemoglobin, it is a fairly stable union of carbon monoxide with haemoglobin instead of oxygen or carbon dioxide. It has a much higher affinity for hemoglobin than oxygen, and at high levels impairs the normal transport of oxygen by the blood. The normal value of CoHb in blood is 3%, but it can get as high as 15% in smokers. Levels change significantly, both in the mother and fetes during maternal smoking, and may lead to chronic intrauterine hypoxia and reduced birthweight. However, it is still not clear if the physiological effects of maternal smoking on fatal growth are due to decreased availability of oxygen as a consequence of the formation of COHb, or to the vasoconstrictive properties of nicotine.
See Hemoglobin, Hypoxia, Intrauterine growth restriction or retardation (IUGR), Nicotine, Vasoconstriction