I painted 5 rooms in our house in my second trimester - just keep the room really well aired while you're painting and stop if you feel unwell. The fumes aren't the best thing in the world for you but painting a room with standard paint shouldn't do the baby too much harm. You can buy a particular paint with less/no fumes but I just went with the standard stuff.
We had to paint our bathroom as it was mouldy and we just chose a Saturday and I stayed well out of the way in living room while OH painted. I think main thing is to keep the house/flat well-ventilated. I would start early in the morning so windows can all be left open as long as possible and just avoid the room.
I'm 35+2 and have just painted 2 rooms. We used a low- odour paint and kept all windows open to keep plenty of air coming in, you couldn't pick up any smell or fumes from the paint. I think as long as you're sensible it's safe. It's a good idea to sleep with the windows open or even better, sleep in a different room while decorating another.
I painted when pg infact we renovated a whole house between me and dh from lead pov, it can be dangerous if there are layers underneath wall paper etc in old houses. We had lots of it and believe it or not arsonic on the wall paper
Buy a mask. 3M make a good one. Have a look on google, you can get a "painting one" or for different things, for example if you were sanding wood. We have a huge decorating restoration project at our new house and I am also pregnant. The deal is we work with the window open if we can and always wear the mask keeping the door closed from the rest of the house.
Paint fumes and sanding dust should not imo be taken lightly. The mask was no more than £30 and I value my lungs a lot more than this.