Excessive histamine can induce uterine contractions and has historically been shown to trigger pregnancy loss in animal models.[16] The placenta normally produces significant amounts of diamine oxidase (DAO), an enzyme that metabolizes histamine, forming a metabolic barrier that protects the fetus.[16] Decreased DAO activity or enhanced histamine release may disrupt this balance.[16] Indeed, reduced DAO activity has been noted in conditions like threatened miscarriage,[16] indicating that an excess of histamine may be harmful. This evidence concerns the gene DAO and spontaneous abortion.