How can I prevent nosebleeds in pregnancy?

Depends. Depends why it is bleeding. Often nosebleeds occur from the nose drying out. Using saline spray o keep the nose moist may help prevent nose bleeds. If your nose bleeds occur from the front near the nostrils, over the counter aqueous gels may help as well. If they continue, you should have some blood work done and possibly see an ENT doctor and get an nasal scope.
Moisten nares. 1st make sure blood pressure is normal. Most common cause of nose bleeds in pregnancy are the combination of nasal congestion and dry crusty nose in the am. Using a small amount of vaseline or moisture cream in opening of nasal passages at bed time helps.