You can loop a rubber band through your button hole and attach it to your button. Add more rubber bands as you expand. That was the 'old school' way of doing it before bella bands.
Stretch pants are making a comeback and those work so well during pregnancy! Obviously because they are stretchy in the tummy area.
Sweat pants. But those get old and boring, I know.
You might try some maternity pants on. Maybe they would feel better.
I didn't start wearing maternity clothes until I was about 6 months. But a friend of mine started when she was 9 weeks. Everyone is different.