One of the problems with being pregnant is that nothing fits. It doesn't matter if it is maternity clothes, regular clothes, or clothes a size or two too big. You will spend the last few months of your pregnancy hiking stuff up, creatively using safety pins, and covering your chest with folders to hide your goods.
Tops. My maternity shirt is gaping, showing off the girls to the point of distraction for a few people. I don't know why manufacturers insist on making crisscross tops and v-necks for pregnant women. Only worse are button-downs, which I thought were cute the first time I was pregnant, until I realized that no matter what they were not going to lay flat and I would spend most of the day flashing unsuspecting colleagues.
Bottoms. Maternity pants don't stay up. The ones with big stretchy sections roll off your belly (or at least mine, but it could be because I am carrying kind of pointy), and the ones with bands that fall below your belly inch down when you walk. And, if you are petite like me, don't be a bonehead and get your pants shortened early in your pregnancy. By the time you get to the end they will be too short and you will look like you are wearing highwaters.
Undergarments. My panties are in a bunch. Literally. They keep falling off my butt and bunching up in the back. I look like LP with a full diaper (and a much bigger arse). Maybe this is because I am wearing my regular underwear, but for some reason the maternity underwear only come in bikini briefs; I don't wear bikinis when not pregnant, and I can't understand why someone would want less material when they have more to cover. And don't get me started on bras. I have four different sizes in my lingerie* drawer. Even ones that are labeled as being the same size don't fit the same, even from the same manufacturer.
*I don't really own any lingerie, but it sounds nicer than "the bedside dresser where I cram in pantyhose, socks, bras and granny panties".