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In many cases, you can allow the pregnancy to proceed normally, but make sure you have a vet on hand in case there are complications, which are more likely if your pig is older or younger or has not given birth previously. Try to find a vet who specializes in rodents and other small animals, rather than just a generic vet. If you have multiple sows, remove the male pig immediately to prevent others from becoming pregnant. Even if this is the only sow you own, you should still remove the male pig before she reaches 60 days in her pregnancy. It is best to house the male in an adjacent cage where he can still be close to the female.  A male guinea pig who can not see or hear his mate may develop stress, making him susceptible to illness.  Male guinea pigs will continue to mount pregnant sows, which can cause stress or pain to your pregnant sow late in the pregnancy (after the 50-day mark). She could also become pregnancy just two hours after she gives birth. You'll want to make sure your pig is getting sufficient food, water, and nutrients since these things are also helping the fetuses develop.  Feed your guinea pig alfalfa hay instead of timothy hay so that she gets more protein and calcium. Your pregnant sow will also need more vitamin C after 4 weeks, about twice as much, so incorporate more fresh fruits and vegetables high in vitamin C into her diet. In addition, you may want to increase your guinea pig's fiber intake. Increased fiber intake can prevent hair thinning, which is common in the last stage of pregnancy. You should weigh your guinea pig twice a week to make sure that is gaining weight (and not losing it) and is generally healthy (e.g., eating all her food, still social and interactive, etc.) and check she's healthy. If at any point her weight begins to fall or if she begins to show signs of illness, consult a vet immediately. Try to make your guinea pig's life follow a routine so as to minimize stress, which can aggravate the dangers that accompany guinea pig pregnancy.  Avoid making changes to a pregnant sow's cage, like removing toys or putting the cage in a totally new location. This could increase her stress and affect her eating and drinking habits.  Reduce her exposure to loud noises or bright lights, including direct sunlight. Reduce handling to a minimum and don't handle her within two weeks of the birth. Note that the gestation period is usually 66 - 72 days.

Summary:
Have the information of a vet. Remove any male guinea pigs. Make sure your guinea pig has enough food and water. Weigh your pregnant sow regularly. Minimize your guinea pig's stress.