Do dogs know when your pregnant?

My 4 year old dog has always been protective of me. My husband and I cannot sit next to each other without him getting a little jealous. Well last night, my husband sat next to me and the dog jumped up on the couch between us, leaned into me, and snarled at my husband. He even barked at my husband, which he only barks when there is a threat (very very rare).

We have been TTC for 10 months. Currently in the 2WW of my cycle.

I have heard stories of pets becoming protective of the soon to be mommas, but how soon do they know???