How much weight does a pregnant guinea pig usually gain in pounds? Im not sure if my piggies are pregnant or not. If they are pregnant they are about 7 and 9 months old and have never delivered a litter before. I heard it would probably be fatal for my 9 month piggy and that just cant happen. If they are pregnant they would be 60 days today. Somedays my older pig looks bigger than others. I guess it depends how she is sitting or lying down.