I don't know. It may be because you might smell different to a dog when you are pregnant, as dogs' sense of smell is something like 50 times more sensitive than ours. Or your puppy senses there is something different about you. Perhaps you have been giving him less attention? And he's trying to get your attention?
I don't know if dogs do bark at pregnant ladies, generally. But yours is barking at you. This cannot go on. If he's doing this now, he'll be like a little devil when the baby is born! You will have a hard time with a crying baby and a barking dog!
This puppy needs training. He needs to see you as Ruler of the home, his Pack Leader. The person who is in charge.
I'm not the best person to advise you on dog behavior. You could take him to a behaviorist (your vet could recommend one I'm sure) or if that's too expensive for your budget, buy a good book on dog training, or get one ordered through your library. (There are bound to be good websites online for this kind of thing) Then follow the instructions. Work with your dog. He loves you and you love him. You have to, or the future may be unhappy for all of you.
Excellent advice, Ginger!
Pandu31, you need to find yourself a good trainer, and do it quickly, because if your puppy is biting you and your brother, you need to end this situation NOW before your baby comes along to be a potential victim.
Whatever the reason is, you cannot allow this situation to continue.