Board Thread:Naruto Gaiden/@comment-27829630-20151228101514/@comment-5936871-20151228220933

I assume Orochimaru impregnated a woman, possessed her body (Just look at how young and feminine he looks in Gaiden), gave birth to Mitsuki, and then sent him to Konoha. Making him both his mother and father at the same time, and thus just his "parent".