[header] How to determine the sex of a guinea pig [title] Do not examine cavies who are younger than three weeks old. [step] Newborn cavies can be rejected by their mothers if they are handled by humans, as your smell will end up on their coats. Newborn cavies can also become easily distressed when away from their mothers, leading to a very difficult time trying to examine their genital area. 
[substeps] Many baby guinea pigs can get cold quickly and may become too cold during the examination. If you feel you need to examine a baby guinea pig that is three weeks or younger, you should try to do it quickly and gently.