It has been known for years that the placenta transfers oxygen and nutrients from the mother to the fetus. But what Bonnin
and his colleagues have learned, through rodent research, is that the placenta also produces the neurotransmitter serotonin
and releases it into the fetal bloodstream where it can reach the fetal brain. Thus, serotonin manufactured by the placenta
putatively supplements whatever serotonin neurons the fetal brain makes.