Although my low-rise jeans still technically fit, my dress pants have been unbearably tight since the end of my first trimester. I had it in my head that I shouldn't wear maternity clothes until at least the second trimester, but then I had a day at work where my unforgiving waistband just made me angry, and I knew that I had to put them away. Out came the stretchy pants.
When you're bloated and nauseated, wearing comfortable, loose-fitting clothing is a must. Maternity pants feel like yoga pants but look like dress pants or jeans or whatever kind of pants suit the occasion. They are meant to be stretchy enough to fit you at every stage of your pregnancy including postpartum. Based on my experience thus far, I think this will be the case. I bought one pair of pants early on, and two more since, and they fit as well now as they did when I first acquired them, despite my expanding belly.
With the relief you feel when you start wearing maternity pants, you wonder why anyone wears pants without a big elastic waistband. Recently I've realized why.
Maternity pants feel good because they're not constricting around your waist. Unfortunately, not being constricting means that there isn't much holding them up. So I spend a lot time hiking up my draggy drawers. I guess you have to have some incentive to get back in your pre-maternity jeans...