They need to be being fed alfalfa hay in addition to the timothy now that they're pregnant, and they'll need the extra calcium. I'd also recommend against feeding any pigs Gerty Guinea Pig, as it's full of seeds and other choking hazards. Pig pellet should be plain, only pellets, with no seeds, colored bits, etc.
Make sure they're getting 1 cup of fresh vegetables per pig per day. Also line up a good exotics veterinarian, as if they need a c-section or something goes wrong, you've got a very short time period to get them medical care before there are problems.