Dog is acting strange

I'm due July 17th and my dog (Australian Shepherd) for the first time ever started barking incessantly when I put him up for the night. It freaked me out, because he only barks like that when he sees someone on or close to our property. Could he just be nervous about changes in our house? Or does he sense my body about to go into labor? Did anyone have an odd experience with their dog just before labor?