Pregnant women with asthma are at risk for having a baby with reduced growth, but only if the mother is not treated with inhaled steroids and is carrying a girl. This holds true regardless of how severe the asthma is or whether the mother smokes.

The reduced growth seems to occur because some unknown factor produced by a female fetus increases inflammation in the mother. If not treated with inhaled steroids, this inflammation impairs the growth of the fetus. By contrast, pregnancy with a male fetus seems to have a weaker effect on maternal inflammation. This study provides evidence that in pregnancies complicated by asthma there is a fetal sex-specific effect on the maternal immune system with adverse effects on placental function and female fetal growth.
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