Carboxyhemoglobin concentration in fetal and adult circulations as a function of CO concentration. When a pregnant woman is exposed to carbon monoxide, her fetus will eventually come to have a higher (COHb) level than her own (maternal) circulation. Initially the rise in COHb in the fetus lags behind that of maternal COHb, since the CO has to pass through the maternal circulation before reaching the fetal circulation. Eventually, however, the higher affinity of the fetus' hemoglobin (a mixture of fetal Hb and adult Hb) for CO means the fetal COHb level will exceed the maternal COHb level. For this reason and others, the damaging effects of CO on the fetus are likely to be greater.
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