I had fun wearing my husband's undershorts.
I tend to buy certain clothes a little larger because I don't always want a snug fit. Those clothes grew with me during my pregnancy. Maybe you can get your pants in a larger size and just expect to add looser pants to your post-partum wardrobe.
Also, start wearing more dresses--maxi or other loose fits that can grow (and shrink) with you.
I didn't buy maternity clothes until about six or seven months because I was tiny, and I thought the same as you about wasting money on something I wouldn't wear long. But I got tired of pulling down my shirt all day. (I still wear the tank tops today.) When I put on my first pair of maternity jeans--OHH!--I wished that I had discovered them sooner, if only so I could justify wearing them longer. I wasn't uncomfortable before, but the belly and back support gave me wonderful comfort that I had no idea that I was lacking! And you know how elusive comfort can be during pregnancy. I also wore those pants for several weeks after I had my baby.
They make maternity clothes to grow with you, so you wouldn't have to buy something new with every spurt. Just buy pieces that fit you in a way that would allow you to continue to wear them. And don't buy the most expensive things out there. There are reasonably-priced maternity clothes out there. I got mine from Target, and a friend shared some dressy pieces. You can go to a thrift shop.
Oh, another thing that helped me was my maternity footless tights/leggings. I wore those with everything that I thought looked good with them. They helped to pull some of my looks together, especially during cooler months.