What age should i take my Babbie guinea pigs to the vets to be ed?

my guinea pig is pregnant and i want to know what age i should take the Babbie's to the vets tot have them ed. what age can you see a clear difference so that the vet doesn't get it wrong and we don't end up with more pregnancy's? thanks xx ( and can guinea pigs have runt in their first litter of babbie's and if so, do they live for a shorter amount of time? thanks) x