As Classylady says, you shouldn't need to do anything. I got two young female guinea pigs several years ago, they were my first ones and I was assured they had been separated from the males before they were sexually mature.
As the weeks went by I thought I must be overfeeding them because of how fat they were getting!
Just as I started to think uh oh, I got up to find that two had become eight!
No need to worry, just coo over the babies if they are born while in your care:-)