The following is a rough guide for care of pregnant "sow" and her babies, before and after birth.

Birth
Early into the second month of her term, the sow will start to become noticeably plump. As the babies grow in size, you will actually be able to feel their bodies and even estimate how large of a litter she will have. When the babies start kicking and moving inside her womb, she is roughly three weeks away from giving birth, give or take a few days.
You won't get many warning signs as the birth day approaches. Although some sows become noticeably "grumpy" in the last week, this is not true of all females. There is really no way to predict when she will go into labor. Fortunately, you do not need to do anything for her to prepare for the babies, other than make sure she has plenty of soft bedding to nest in.

Most births tend to take place around dawn or dusk, which is when guinea pigs are most active. If you are fortunate enough to be present for the birth, you can watch quietly without disrupting the process. If the male is still with her, and he hasn't been neutered, then you should remove him immediately, as the sow will come into season within an hour after birth. Babies are typically born five minutes apart, and the entire litter should deliver within a half-hour. If it goes on any longer than this, it could mean that she is having trouble giving birth, and you will need to get her to a vet right away.

With each contraction, the mother will push the babies out, one at a time. When they emerge, she will reach down and, with her teeth, break the sack that covers the baby when it's in her womb. It is very important that she do this, or the baby will suffocate. After the entire litter has been delivered, she will pass the placenta, part or all of which is then eaten. There is very little "mess" left over after a guinea pig birth; all that will be seen are a few spots of blood here and there on the bedding.

The mother will then lick the babies clean. If the father is present, he may assist in this duty. Afterwards, she will direct the babies to a corner of the hutch, where they will huddle beneath her for warmth.

As mentioned above, within an hour after birth, the sow will come into season and will be ready for mating. You should never allow a sow to become pregnant so soon after giving birth, as it is extremely hard on her body to go from labor straight into pregnancy again. Most breeders recommend waiting at least two months to allow the sow to get back to her normal weight.

Caring for young cavies
Young cavies are almost fully independent of their mother at birth, requiring only her milk and her warmth. They are born with hair, teeth, claws and open eyes. They will begin to run around on their own within hours, and will start eating solid foods within a day. Every two hours, they will line up under their mother to nurse; though sows only have two teats, there is no fighting at the milk line since the babies can munch on hay, pellets and even drink from the water bottle, if it's in reach.
Like other animals that are born precocial, guinea pigs form their social bonds shortly after birth. It is important that they have human contact during this time to socialize and bond them to people. Unlike many other small mammals, the mother will not reject the babies if they are handled by humans, so the more you hold, pet and interact with them, the more they will grow up friendly towards, and less afraid of, humans.

The babies will continue to nurse for anywhere from three to four weeks. During this time, they will follow both the mother and father around the cage. During exercise time, the guinea pig family will tend to form a "guinea pig train", with the father or mother at the head, the other parent at the end and the babies in the middle. This lineup helps ensure that no young ones get lost while moving about. They will chirp and grunt to each other often, and if a baby does get separated from the group, he or she will squeal loudly until one of the parents comes over to lead him or her back to the family.

As they start to wean during the third week, they will nurse less often from the mother, and will begin to be more independent. By four weeks of age, all the babies should be completely weaned, and no longer require milk from their mother. At this point, you should start separating the males from the family. Males can become fertile at any point after weaning, and will be capable of mating with their mother. Females can come into season any time after one month of age, usually at 5 weeks, and can then be mated by their brothers or their father. Hence, separation of virile males and females is extremely important to prevent uncontrolled inbreeding.

After they have weaned, you may safely give away pets to new homes. For the first day or two, the baby will probably be somewhat frightened of its new environment, having lost the security of it's family and it's familiar hutch. Time, and lots of love and attention from the new owners, will be needed for the baby to adjust to it's new surroundings.

Check out these pictures below!

Am I adorable or what?

Mama Pig with 3 babies!

Mama Pig with 3 babies!

Babies nursing just minutes after being born!

Close up of baby being held by lisa.

Baby is not quite a day old!

Another picture of a baby pig, held by Lisa.

Baby guinea pig, just a few days after birth!

Cute little pig, don't you think?