Not sure if you can really do anything to prevent them, though keeping your nasal passages moist can help a bit. I suffer from nosebleeds normally, so I've had really bad nosebleeds during my pregnancy. I take Kleenex with me everywhere, just in case!
I had a lot of sinus trouble the first tri and now it's coming back again in the 3rd. My doctor suggested using the ocean saline nose spray (it's over the counter) and I use that twice a day when my nose is really dry. When it gets really bad he also gave me an Rx for a nose spray that I use at night. Good luck!!
It's pretty normal but make sure you mention it to your OB. I had never gotten them my entire pregnancy and at my 38 week appointment, I had gotten one that morning. She ran a CBC just to be sure my platelet count was ok. Combined with other symptoms, it can be an indicator of pre-e.
If I am in AC too much I get nose bleeds, pregnant or not! Hard to stay out of AC when I work in an office all day. I found a humidifier in the bedroom at night really helped along with the saline spray others recommended. I would mention it to your doctor also just to be on the safe side.
Not sure what to recommend. They are very common though. I have had them pretty frequently for most of my pregnancy, lucky for me though, they seem to stop pretty quick. I bet a humidifier by the bed at night would help. I personally get mine mostly at night.
Re: Nose bleeds
-My step-daughter is 12 years old.
-BFP #1 on 9/2/12, D&C 10/18/12 no heartbeat on US @ 10 weeks.
-BFP #2 on 1/7/13, R was born on 9/22/13 via C-Section
I would mention it to your doctor also just to be on the safe side.