Guinea pigs are capable of breeding at just 3 to 4 weeks old. This makes it very important to make sure you know which sexes you have before putting them in the run together.
Female guinea pigs have a little triangle or v at the front of the anogenital opening, while males have a circle with a cross on it (a bit like a hot cross bun!)
A baby Agouti guinea pig
Guinea Pig BreedingIf you do intend to breed, the timing for the female is very important. The earliest age is 5 months, and she should be bred from before she is 10 months old. This is because after this age the bones in her pelvis fuse, and make it more likely that she will have problems delivering her babies. If she has a litter before 10 months of age her pelvis will stretch to allow for childbirth, and she shouldn't have problems with any further litters when she is older.
Baby Guinea Pigs
Guinea pig gestation lasts a long time compared to many small mammals - 68 to 70 days. This means they are born at an advanced state of development (a far cry from the blind, pink, helpless babies of rats and mice!) They are fully-furred and have their eyes open, and within a few hours they are able to run around. They look like miniatures of their parents. An experienced guinea pig keeper will able to be able to sex them immediately - seek advice if you're uncertain. If you don't want to be overrun with baby GPs, you need to sort out the gender issue before they are 3 weeks old.