The maternal uterine blood vessels and the fetal umbilical blood vessels run in parallel and oxygen is exchanged passively. Consequently, fetal oxygen pressure can never exceed maternal venous oxygen pressure. But babies adapt to lower oxygen levels. In light of the low oxygen tension of blood delivered to the fetus, other mechanisms must be used to enhance fetal oxygenation. In this respect, fetal hemoglobin plays an important role.
Fetal hemoglobin has a higher affinity for oxygen relative to adult hemoglobin.
Further, fetal hemoglobin is more highly concentrated as compared to adult hemoglobin, which increases the oxygen carrying capacity of the fetal blood. An additional method enhancing fetal oxygenation is the increased blood flow to some fetal organs compared with adult organs. Blood flow to select fetal organs can be greater than twice that of the adult organs. In regard to the safety of asthma medications, in a large study of 824 pregnant women with asthma, no significant association was found between use of most common asthma medications during pregnancy and adverse perinatal outcomes. Specifically, inhaled beclomethasone dipropionate, metaproterenol and albuterol were not found to be associated with congenital malformations.