Studying the effect of maternal asthma during pregnancy on placental function and fetal development has highlighted that there is a strong interaction between mother, placenta and fetus and these interactions appear to be sex-specific. This work has found that the female fetus alters maternal asthma during pregnancy by upregulating maternal inflammatory pathways. When asthma-associated inflammatory pathways are not treated with inhaled steroids during pregnancy, the female fetus has reduced growth and adrenal function due to alterations in placental glucocorticoid metabolism. When the mother uses inhaled steroid for the treatment of her asthma during pregnancy, female fetal growth and placental function are comparable to the control population. The growth of the male fetus appears to be unaffected by asthma or inhaled steroid use. These findings indicate there may be different mechanisms regulating placental glucocorticoid and immune mechanisms depending on fetal sex in both asthmatic and non-asthmatic pregnancies.