Female guinea pigs can be sexually mature as early as 4 weeks old. Gestation is from 59 to 73 days and average litter size is 1-4 but can be as many as 7 or more.
Breeding guinea pigs is not recommended.
Not only is it risky, it is difficult to find homes for the young with responsible and caring people -- your candidates may "disappear" when the time comes to adopt out the babies.
Guinea pig sows are at risk of pregnancy complications because the babies are born large and ready to run. Breeding after 8 months of age can be fatal for a guinea pig who has not had a previous litter due to dystocia. The symphysis (a joint of tough fibrous cartilage which firmly joins the 2 pubic bones) can stiffen upon reaching adulthood and she may not be able to deliver her pups unaided. Sows with dystocia usually need a caesarian section. The survival rate is very poor.
Spaying or neutering guinea pigs also carries risks even when performed by an experienced guinea pig veterinarian. The safest choice is to keep the sexes separate or have only sows or boars.
Should you find yourself with a pregnant guinea pig, read over the advice and links on the Reproduction Section. Since most sows will have an estrus (a time when they can become pregnant) from 2 to 15 hours immediately after giving birth, remove any boar from the cage as the delivery date approaches to prevent back-to-back pregnancies.
Breeding Your Pet Guinea Pig -- Advice For Pet Lovers
Make no mistake: every pregnancy carries significant risks.
When I was first researching guinea pigs to see if they would make a good pet for my daughter, I found only one friend who'd ever had guinea pigs in her home. Her daughter had a single sow she loved and cared for. This sow was bred to a friend's boar. The birth was uneventful, the pups a joy. She bred her a second time with tragic consequences, losing the sow and all the pups and leaving her daughter devastated. Similar stories are repeated over and over again. But the ending is the same. Will the next sad story be yours?
Death is something breeders get used to. One ARBA judge estimates that complications from breeding will kill one in 5 sows (see Cavy Spirit). Guinea pig sows are especially at risk because the babies are born large, fully furred, and ready to run. The mother generally carries several pups and her weight may double, putting stress on her circulation system and other organs. Even with the best care, sows can suffer from dystocia, hypocalcemia, a prolapsed uterus, or pregnancy toxemia.
"What about the pups?"
If you are not concerned for your sow, consider the lives of the pups. If the mother dies, will you have the time to hand rear the pups? Not only is breeding risky, it is difficult to find homes for the young with responsible and caring people -- your candidates may "disappear" when the time comes to adopt out the babies. What if it is a large litter of 6 or 7? Who will care for them? Will you take responsibility for their health and well being? Can you afford the cost of food, shelter and medical care? What if the pigs you breed carry defective genes and your pups are born with congenital problems? Mis-sexing a pair of pigs meant unexpected pups with heart problems (See: SusieQ's story). Do you have space for additional cages? Will they be dumped on a pet store or overburdened rescue? There are far too many guinea pigs and far too few good homes. Given the large number of guinea pigs already needing homes, the responsible pet owner will not add to the population.
So you say you're going to be a "responsible breeder"? Read Josephine's "The Responsible Breeder" at Cavy Spirit to see how you compare. Some breeders cannot reconcile the fact that their choice to breed results in the deaths of sows and their young, and finally stop breeding and showing altogether.
Pet lovers and breeding simply don't mix. But sometimes accidents happen. Two guinea pigs guaranteed to be "of the same sex" are anything but. Your babysitter accidentally mixes your carefully separated animals. Or you buy a pregnant pig from a pet store. Too frequently, pet stores mix sexes and the unwary customer may take home a pregnant female. Perhaps you are fostering a pregnant rescue pig. In any case, you do the best you can, provide good nutrition, separate the sexes and prepare for the birth. Young guinea pigs can have successful pregnancies, but it is never recommended. The advice on this page is here to help those whose pigs are pregnant through no fault of their own.
Because guinea pigs rely on us completely for their care and housing, we can control their reproductive lives and make a conscious choice to be a responsible pet owner. We are fortunate that our guinea pigs are not unspayed cats or dogs, free to run through the neighborhood and become pregnant through our inaction. For the health and well-being of your or your child's beloved pet, do not breed.