"His injuries were concealed when they should have been revealed - that is the essence of the charge against you."
While the Crown could not prove how the injuries were caused, Judge Adeane said there was "a powerful probability that one of you caused them".
"They are not the outcome of some mere domestic mishap."
The offending was further complicated by the fact the baby was, at least biologically, the unwanted child of an acquaintance or family member. Despite this, a birth certificate had been issued indicating it was the couple's child.
"The child in question was a 10-week-old baby who was in the exclusive care of the two of them," the judge said. "While he has been referred to by them as their son his true position is more complicated than that."
The injuries occurred sometime between when the baby was born on January 26 and March 25, in 2011.