Space-filling model of the hydrogen sulfide molecule (Ben Mills)
A recent study (here) indicates that H2S may play a role in regulating blood flow on the fetal side of human placenta. One of the enzymes that generates H2S (CSE) is found in the smooth muscle layer of stem villus arteries. Moreover it was shown using the H2S donor NaHS that the molecule is able to dilate the fetal placental circulation. Because the placenta lacks innervation, signaling molecules like H2S are likely to be important in the regulation of blood flow. The study found evidence that CSE expression is decreased in pathological pregnancies such as preeclampsia and fetal growth restriction.
H2S is involved in oxygen sensing in the carotid body (here) and one wonders if it might perform a similar role in the placenta.