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Anna, Pet Expert/Biologist
Category: Pet
Satisfied Customers: 11464
Experience:  40 yrs.: herps, pocket pets, rabbits, poultry, dogs, horses. Biology degree. Vet assistant.
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I (thought that I had) bought 2 sister guinea pigs last week.

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I (thought that I had) bought 2 sister guinea pigs last week. One of these sisters, I have just figured out, is actually a brother. "Lillybet" has been dancing around "Rosebud" and making purring sounds. I thought they were just playing, until a few minutes ago. "Lillybet" tried to mount "Rosebud". I don't mind this mistake in gender, but is it okay for brothers and sisters to mate in guinea-pig-land? Also, both guinea pigs are only about 4 inches long, so I assuming they are about 3 months old(?). Is it even safe for "Rosebud" to become a mother at this age?

It's too bad this mistake occurred. Male guinea pigs reach sexual maturity at an earlier age than the females. It would be better for Rosebud not to become a mother until she is 5-6 months old. You wouldn't want to wait any longer than that, because giving birth after age 8 months is very dangerous for guinea pigs. I recommend that you separate the guinea pigs for a couple of months, even if you want babies.

Any time you mate a brother and sister, there is an increased risk of the babies having health problems or defects. That doesn't mean it happens every time, but that the risk is greater. There are other things to consider before breeding. It is estimated that about 1 of every 5 female guinea pigs dies while giving birth. Being over 8 months old increases those odds.

I'll give you some additional information on guinea pig reproduction. The gestation period is 59-73 days. Baby guinea pigs are unusual in that they are born covered with hair, eyes open, and able to run around. They're quite adorable. Because the babies are born so developed, female guinea pigs are more likely to experience birth complications than many other mammals. It would be good to find a vet ahead of time who would be willing to help if assistance is needed. Not all vets treat guinea pigs, so you may have to call a few. Chances are all would go well, but it's good to be prepared. This link will take you to a directory of guinea pig/rabbit vets:

The litter can range in size from 1-7 babies, with 2-4 being most common. The babies can be weaned between 2 and 3 weeks of age. By 3 weeks, many males are able to breed, so be sure to remove any male babies to a separate cage by that age. For more about guinea pig reproduction:

If your guinea pigs are primarily pets, I wouldn't take a chance on breeding. If you have a large cage, you can use a divider to separate it into two sections. That way the two can still socialize without having physical contact. You can accomplish the same thing by setting two cages side-by-side. While rabbits are routinely spayed and neutered, guinea pigs usually are not, at least in the USA. The reason is that they have greater risks from surgery. If you'd like to look at the option, neutering the male is less risky than spaying the female. I would consult with a vet experienced with guinea pig surgery. If you have any further questions about this, just let me know by clicking on REPLY.

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Customer: replied 8 years ago.
I am so upset. I have grown to love the little guy... Financially I can not afford his neutering surgery, so I guess I will have to return him. Thanks for all your help, though.
I'm sorry to hear that. Are you sure you can't find a way to separate the two without spending very much money?

Please don't click on accept again. Thank you for accepting above.

If I can help you in any other way, let me know.