Maternity underwear is usually made to be extra-stretchy and either low-cut, to fit underneath a growing belly, or high-cut, to fit over it. But no, it's not a necessity. You can totally get away with wearing some regular comfy undies, as long as they have plenty of stretch and are low-cut. Just beware that you may have to buy some in a larger size late in the pregnancy. Luckily, it's easy to find soft, durable pairs that are affordable. (Think Hanes!) Look for the microfiber seamless kind—they're lightweight and won't add bulk to your frame.