I don't see how this proves smoking cannabis during pregnancy results in harm to a developing fetus or infant. It just says "no one knows how excessive exposure to the hormone might affect a fetus or infant". Also, that babies "have to rely on their Mother's Breast Milk to get it" indicating the infants would acquire the hormone anyways through breastmilk (assuming the mother feeds the infant that way).
To use this as the basis for the argument that smoking marijuana at any time during pregnancy is a bad thing would assume that excessive exposure to melatonin negatively affects the fetus.

As for this Italian melatonin study, I don't doubt it but a study involving only eight men doesn't totally satisfy my skepticism.