I have had dry mouth thru out my whole pregnancy. Some things I find that help is using this mouth wash that is reccomended to diabetics my dad uses it anyways it was in cupboard at home so I use it now and it really helps . the mouth wash is called biotene you can get toothpaste too . I didn't find the tooth paste worked as good as the mouth wash. It is alot more expensive then regular mouthwash but it helps. What also helps is taking a glass of water and leaving beside your bed at nighttime. Thoses are a few of the things I been doing that help. I get dry mouth during day too so I drink more water and suck on mints.
It could be because of nasal congestion. You may be sleeping with your mouth open.
It's been happening to me half of my pregnancy I'm 35 weeks pregnant and it's still happening. I keep Gatorade by my bed at night unfortunately it means more late night bathroom trips.
Yes I'm 35 weeks Friday and wake up every few hours in the night for a drink.