The pregnancy of a guinea pig is quite different to that of a rabbit ....

knowing the exact date of conception to the date of birth is hard to tell as it depends on the length of the guinea pig's cycle to how many babies she is carrying. This may sound strange but a guinea pig pregnancy can actually vary from 56 day to to 75 days - this will be affected by how many pups she is carryings. So it's not an exact science!

Basically they arrive when they arrive!