Hemorrhoids are caused by pressure in the hemorrhoid veins. There are several reasons pregnancy may cause extra pressure. For one, a pregnant woman's enlarged womb increases pressure in the hemorrhoid veins. Secondly, a pregnant woman's blood supply actually increases during pregnancy, raising the pressure within her veins. Lastly, a pregnant woman's hormones tend to relax the anal and rectal area's supporting muscles as the baby presses more and more on the veins below it. All these can create extra pressure that triggers hemorrhoids to flare up or form. Add to all of this the fact that pregnancy often causes constipation (a trigger of hemorrhoids), it is no wonder why expectant mothers get hemorrhoids. If you had hemorrhoids before pregnancy, you're more likely to get them again while pregnant. They may also develop or flare up with the increase of pressure caused by straining during labor. For more information, see Just for Moms.