I was having a discussion with my girlfriend.?

We were discussion what gymnast are supposed to wear under the leotard. I say that they are not to wear anything under it as a leotard is similar to swimwear. My girlfriend however claims that there should be some type of underwear worn underneath. I believe that if anything but a high rise thong or g-string was worn it'd show on the sides. Please help us solve this dispute and give us a suitable answer. If a gymnast could answer this it'd be very helpful.