What causes nosebleeds in pregnancy?

Fragile capillaries. Due to the high estrogen levels in pregnancy, the small blood vessels in our mucous membranes (nasal areas, oral areas) can become fragile and easily irritated --- which is why pregnant women may see nosebleeds or bleeding gums more frequently.
Vascular dilation. During pregnancy, your blood vessels in your nose to expand, and your increased blood supply puts more pressure on those delicate vessels, especially in the front, causing them to more easily rupture and bleed. Blood vessels can rupture as well in infection with cold, sinus infection, or allergies, when the membranes inside your nose dry out, trauma, high blood pressure or a clotting disorder.