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 stretches 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.
Source: http://www.maternityfolks.com/10-best-maternity-underwear-for-new-moms