I think Jess made a fantastic point - i'm not a mum yet but i can completely understand your want for just you time.
I'm the least social being on the planet at the moment, i hate the idea of having to go out to do the grocery shopping and im cursing if i've forgotten something when i get home. I have the freezer so full of food so i can just make DF pop out for me every now and then. I've been refusing to answer my friends phone calls and the only people i want around is my mum and my anthony.
Pregnancy is such a personal time and you shouldn't feel like you have to listen to what ANYONE else says about your social graces, and like you said, you will know when you have gotten to the point that you might need help for PND, iykwim? I am sick to death of people suggesting this and suggesting that when quite frankly i'm content. Tell them to go shove it - & then blame the hormones if they get upset :P
Good luck with the rest of your pg
xoxo