It goes without saying that your body will change during your pregnancy. And it is not just the growing 'bump' that will alter, as you go through the pregnancy you may find a lots of changes you don't expect! Every pregnancy is individual and not all women will change in the same way but most women find they need to reassess their wardrobe.
The first trimester is tricky, it's not advisable to go out and buy lots and lots at this stage because the changes in your body have a long way to go! You are also at the stage of your pregnancy where it is likely that very few people know that you are pregnant, so you are also dealing with trying not to 'look pregnant'.
While you might not have a noticeable bump at this stage, many women experience bloating which can make your clothes feel really uncomfortable. Even though I could still fit into my regular jeans at first, they felt like a torture device so I switched to some stretchy jeggings, one size up for comfort (I preferred the high waist ones). These can be bought fairly cheap and come in handy post pregnancy too. Leggings are another good option as they can bought cheaply, and can be worn under long tops and give support under dresses.
Another good tip is to buy some cheap stretchy maternity bands, which you can use to get a bit more wear out of your non maternity jeans and trousers without having to go out and buy more. They help hold your trousers up without you having to do the button up and cost a fraction of a new pair of trousers.
I made good use of any long t-shirts and shirts I already had, and invested in a couple of loose fitting tops from H&M. They do a really good range of basics at an affordable price and I could slip them into my existing wardrobe. Again, any loose fitting tops I bought have come in handy post pregnancy too.
I also invested in a plain black stretchy skirt in a larger size (see picture) which was much cheaper than a Maternity one (I think it was only about £7, again a H&M bargain) and I was able to wear it quite far into my pregnancy because of the stretch. One or two stretch skirts that can slot into your existing wardrobe can be really handy, especially if you work somewhere you have to wear 'office' wear.
You may have items already in your wardrobe that are cut in just the right way to disguise your 1st trimester bump. I already had some loose fitting dresses so it wasn't suspicious if I rocked up in one. And the bloating plus the knowledge that you have may be making you feel bigger than you actually look so you can probably get away with more than you realise.