Hello,Customer and thanks for your question.
Congratulations on your new cavy family!
I'm so glad you're asking this question now
, because male guinea pigs (boars) should not be housed in the same cage with their sisters or mother (sows) after the age of 3-4 weeks, in order to prevent pregnancies, so you need to separate them as soon as you determine their sex.
Females usually have a smooth swelling over their genital area. It can sometimes look bumpy causing you to mistake it for a male, however, if you gently part the genital opening on a female, you should see a "Y" shaped opening, and then you'll know it's definitely a female (sow).
To check if some of your babies are males (boars), press gently just above the genital area. If it's a male, you should be able to make the penis slowly come out, due to your gentle pressing. However, don't be fooled if you don't see a penis present; sometimes the penis can be tucked away and out of sight, in folds of skin (usually in older males, so with the babies, it should be easier), making it look completely like a female. You should try to get the penis to ease out, just to be sure you have a male. In addition, younger males will have a donut-like shape around their anus, where their testicles are located.
If you're still not sure of their genders, bring your guinea pigs to a vet who is experienced in treating them; don't bring them to Petco or another pet store, as the clerks may not give you the correct information you need. Only a vet who's used to working with guinea pigs will be able to provide the most accurate information on which ones are female and which ones are male.
You can see photos and a detailed description of how to determine the gender of your baby piggies, below, in addition to much more helpful information: http://www.cavyspirit.com/sexing.htm
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Best wishes to you and your guinea pig family!
Edited by Jessesmom on 2/13/2010 at 6:33 AM EST