Sniffing your belly. Backing away from you when you walk. Being more responsive to your cues. Being less responsive to your cues. Staying right near you all the time. Growling at you. All of these are possible reactions by dogs to a pregnant guardian.
I’m often asked if dogs are able to sense when a woman is pregnant. I spoke to Rachel Rounds, a journalist in the UK who was expecting, and she incorporated my answers to her questions into her article “Clingy, need and moody. It’s Rachel who’s expecting – but it’s her dog who’s gone all hormonal.” 
I’m not aware of any research that directly addresses the question of whether dogs know that their guardian is expecting, but it would be very surprising if dogs didn’t at least pick up on some of the accompanying changes and react to them. Dogs can obtain an amazing amount of information about other dogs just from smelling each other or even each other’s urine (e.g. Male or female? Intact or spayed/neutered? In heat? Young or old? Familiar or a stranger?) Given what we know they are able to perceive with their nose, it’s a bit hard to imagine that they can’t detect at least some of the many hormonal changes that accompany pregnancy in a person living in their house.
At the risk of giving too much information, I can detect pregnant urine. I knew of a few friends’ pregnancies before they announced them just because I happened to use a bathroom at a social gathering immediately after them. Since I, a mere human, have a nose for it, it’s more than likely that dogs do, too. Of course, it’s hard to say whether dog know what the change in odor means, but it seems unimaginable that they don’t detect it.
Once a pregnancy is far along, women change their movements a bit, partly because of the normal loosening of the joints, and partly because carrying another person in your abdomen is cumbersome, to say the least. Dogs are very sensitive to movement and posture of the most subtle form in other individuals. That pregnant kangaroo stance and that waddling gait are far from subtle, and cannot be hidden from people or from dogs.
Pregnancy is often accompanied by behavioral changes, and these can extend beyond the woman expecting to other members of the household. Those changes may have to do with the schedule—more sleep, fewer walks and runs, more time spent redecorating—or may be emotional with shorter tempers, conflict, stress, or other issues in dealing with one another.
Most dogs are going to pick up on at least some of the changes associated with pregnancy, and these can certainly have an influence on their behavior. Did you notice any changes in your dog’s behavior when you or someone else in your family was pregnant?