There are a variety of reasons pregnancy causes hemorrhoids. For one, a pregnant woman's enlarged womb increases pressure in the hemorrhoidal veins. Another reason - a pregnant woman's blood supply actually increases during pregnancy, raising the pressure within her veins. Additionally, a pregnant woman's hormones tend to relax the anorectal area's supporting muscles as the baby presses more and more on the veins below it. Added to all of this the fact that pregnancy often causes constipation, it is no wonder why pregnancy causes hemorrhoids.