I bought some maternity leggings off asos. I'm a tall and slim size 8-10 usually. bought the leggings in a size 12. they were too big for me for a while but once I got some belly happening and basically after all my normal jeans became too small, I wore the leggings constantly, they took me right through to birth. I put on about 10kg in total in pregnancy.
they're pretty inexpensive so you could just get a couple of pairs in your next size up and see how you go. the over the bump waistband was pretty generous and leggings are pretty forgiving.
I bought mine in a maternity starter pack and got the leggings, a long line over the bump singlet and a belly band (all in black). it was great, not sure if that's still available on their site though.
I never wore jeans during pregnancy. wore my normal ones as long as they would fit (I didn't get far past 12-14 weeks I think) then just switched to leggings.