Progesterone (another of those pregnancy hormones) is relaxing smooth muscle tissue all over your body, including your gastrointestinal tract. This makes your gut work slower, giving your body more time to snatch up nutrients from your food and take them to baby... and translating into gas for you. Later in pregnancy, your bulging uterus starts pushing up on your stomach and down on your rectum, further increasing risk of heartburn and constipation.
Pain, belching, bloating and other nasties are no fun, but — sorry — they sort of come along with the job of carrying a baby. Luckily, there are ways to relieve some of the pressure. Eat small, regular meals, and stay away from foods that tend to give you gas. Fried foods, sweets, cabbage and beans are common culprits, but you might find other foods that are particularly troublesome. Eating and drinking slowly will keep you from swallowing excess air (you'll later use this technique when feeding baby!), and loose clothing will keep you comfy. Yoga classes can also help settle things down. And, ward off constipation (a big gas-inducer), with plenty of liquids and high-fiber foods.