First of all, it's a good idea not to try to breed a guinea pig
     until you have found some responsible people who would like one
     of the offspring as a pet.  Pet stores often treat small animals
     very irresponsibly, and you don't want to bring guinea pigs into
     the world that aren't wanted or will be mistreated.

     That in mind, there are a few caveats.  A female should not be
     bred until she weighs 500 g, or is 4-5 months old.  Also, no
     older female should ever have a first litter.  Somewhere between
     the ages of 9 and 12 months, if she is childless, her hip bones
     will fuse such that she can not give birth naturally, and a later
     pregnancy will require a caesarian section.  Therefore, if you
     plan to breed your female, or if you do not plan to spay her and
     the situation is such that she may become pregnant later on, you
     should probably see that she has at least one litter between the
     ages of 5 and 9 months.  If an older female does accidentally
     become pregnant with a first pregnancy, you and your vet will
     want to plan on surgery to deliver the babies, otherwise she will
     likely die giving birth.  In addition, do everything you can to
     avoid such an accident in the first place (for example, have your
     female spayed even if you think she won't be near a male), since
     a caesarian section is risky for both mother and babies.  For
     more information, see _Diseases of Domestic Guinea Pigs_ by
     V.C.G. Richardson.

     The gestation period (time between conception and giving birth)
     for guinea pigs is approximately 60-70 days.  Guinea pigs do not
     normally require assistance in giving birth.  The young are
     usually in no danger from either parent, although you may want to
     remove the male right away, since the female is able to conceive
     again within the hour after giving birth.  Litters can have
     between 1 and 8 little ones, but typically have two to four.  The
     males of the litter should be separated from the mother and their
     sisters directly after weaning, since they are sexually mature
     shortly after.  The babies will probably be weaned by the time
     they are about 3 weeks old.

     It is important to handle the babies soon and often, to socialize
     them to humans.  Like other animals that are born precocial,
     guinea pigs form their social bonds shortly after birth, sometime
     within a matter of hours, so human contact is critical during
     this time to ensure that they establish strong bonds to people.
     Many people are under the impression that handling baby animals
     too soon will cause the mother to reject them, but this isn't
     true for guinea pigs.  Lots of love and gentle handling and
     petting from the start will make the babies grow up more
     friendly, and less afraid of humans.