I would say he is reacting to the hormonal changes of your pregnancy and you are right to be concerned as he is acting as if you are lower in the pack ranking than he is.
He is at the age when dogs mature and try to put themselves higher in the pack order.
For home actions on this issue I'd suggest putting the dogs on the nothing in life is free program which you can read about here
http://www.paws.org/cas/resources/fact_sheets_dogs/nothingfree.php
its a non violent way of changing how the dogs see you and turns you into the leader.
I would also suggest seeing your vet to make sure no heatlh issues are impacting him such as a tick carried disease or a thyroid problem as those can cause temperament changes.
While talking with the vet ask for a referral to a veterinary behaviorist. If you intend to keep the dog you need to get this under control before the baby is born and all your attention has to be focused on newborn care.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics