Dressing well during pregnancy is a lot easier now than it was when I had my first child 10 years ago. The options are so numerous at different price points that it's easy to be stylish and comfortable without spending a fortune, and with careful planning you can find clothes that you can actually modify and continue to wear once you've had the baby.
Maternity clothes aren't really needed during the first trimester, although in your excitement about the baby you're likely to want to run out and buy them the moment the little line appears on the stick. Wait a while, really, because by the third trimester you'll be sick of the sight of them, and by the second kid you won't be willing to put maternity pants on until you're ready to pack your bags for the hospital.
Here are a few tips to make dressing during pregnancy easier:
During the first trimester, you almost certainly can get away with wearing regular clothes in styles that accommodate changes to your figure, such as the Empire-waist blouses that are so popular right now. Your breasts will be larger than usual due to hormones, so purchase a well-made supportive bra. A nursing bra is a good investment even this early, as it will be one of the few items you'll be able to continue to use for months or even years after the baby is born.
Feet will swell over the course of the pregnancy too, so if you buy shoes at this time, go for a half-size larger.
The second trimester is when maternity clothes become appropriate. Dresses are a great option for work, although some upscale lines such do also make maternity suits. For casual wear, tops and maternity pants are probably the easiest choice. Maternity pants come in two basic styles: elastic-waist, which expand to fit your belly, and under-the-belly waists, which just hang below the baby bump. Try both kinds on to see which is more comfortable for you, but remember that how you feel the first week of the second trimester compared to the end of the pregnancy may be very different.
Liz Lange has also come up with a line called The Fourth Trimester that is intended for transitional wear after the baby is born, although most of us are resigned to the fact that you may need to keep wearing maternity clothes for a couple of months postpartum. Just make sure you're not still doing it when the "baby" turns six!