they can smell the baby or pregnancy, it is instinctual to protect the weaker members of the
pack.
my dog still hasn't noticed, but she does enjoy staying in bed longer in the mornings.
when the baby is born, be sure to have someone take the very first blanket they wrap the baby in home to the dogs - so they get used to the scent. the first one usually has some blood from the after birth on it, so good string smell.