In my household, I was told as a very small child that non-family members that are on their menstrual cycle are not allowed to hold babies. It was said that the pain that the woman is experiencing in her womb, is transferred from her to the child. In turn the child will get sick, and receive colic.
I think that its true, I find myself doing it at times as well. She also added that she has learned this from her female family members since an early age. She said as well that she learned it from the women on the Venezuelan side of he family, and it was passed down from her grandmother.
I think that this superstition is probably not true, because of the fact that it has no basis in science. It is simply that pain would be transferred from a stranger, which I feel is almost impossible. I believe that it exists merely because of the fact babies come from the womb, and that if someone who is unfamiliar is in pain in the womb. Its just that connection.