You've got extra blood flowing through your veins right now, and it can sometimes pool up in the parts of your body most affected by gravity (such as, yes, the rectum). The result is swollen, itchy varicose veins, and when these come where the sun don't shine, they're called hemorrhoids. Your growing uterus adds pressure to the region, making it particularly prone to swelling, and the forthcoming strains of childbirth can start the problem right over again.
Though hemorrhoids sometimes (sorry) just happen, there are ways to reduce your risk. To avoid trapping more blood in the veins, try not to strain when you're doing your business. Prevent constipation by staying hydrated and eating a well-balanced, fiber-filled diet (which you should be doing anyway). Since excess weight can also contribute to hemorrhoids, avoid gaining more than your doctor recommends, and get off your bottom often — both during pregnancy and after delivery. Moving around eases the pressure on the veins in your pelvic area and gets your bowels moving. When you're feeling lazy, though, lie down and make yourself comfortable — long periods of sitting or standing add hemorrhoid-inducing pressure. Kegel exercises can also ease hemorrhoids by increasing circulation to the area, along with helping stop urine leaks and decreasing the chances of an episiotomy... three pretty convincing reasons to give them a try.