Clothing Hack: Your Growing Baby Bump
While your pants still fit everywhere else besides the waistline, you can use a hair elastic to secure the button through the button hole. This will give you enough room to prevent chafing and scraping on your lower belly. This is the easiest fix that can get you through an extra couple of weeks with your pre-pregnancy pants.
The other option is to buy an inexpensive belly band. This band wraps completely around your midsection to keep your shirt and pants tight and secure to your waist line. In addition, the belly band provides phenomenal support for those muscles that wrap around your ever-expanding belly.
Source: Ingrid & Isabel Bellaband
If you find yourself expanding sooner than the second trimester (normal if you are having multiples or if it is your second or third baby), you can usually find pants with drawstrings that work beautifully as well. They don't usually cut into you and won't cause chafing. They will sit comfortably on your hips.
Try these tricks before running out to the maternity stores for those pricey maternity pants.