maternity clothing

This. Might be a dumb question but how do I tell what size I need do you just got you normal size or should you buy sizes up Iv had 5 kids and never needed them I went from size 6 to size 8 them back to size 6 two days after I had my other Bubs did even show till I was 7/8months 
This time it's twins I'm 19 weeks gone to size 8 but find the jeans or pangs rub and leave a red itchy mark under my beautiful rounded cute belly