How To Tell The Difference Between Male And Female Guinea Pigs?

Ok here is what you do pick the guinea pig up if it is a young guinea pig feel under the butt near the back legs if you feel 2 little balls it is a male if you don't feel 2 little balls it is a female ok if it is an older guinea the nuts should have already dropped so you should see big bumps towards the rump.if there are that means it is a male if not it is a female. There you go now you can sex your guinea pigs.

This is a wonderful site that has very clear picture on how to sex a guinea pig.  Good Luck!
If you have males with your females, the males can impregnate his own mother at two weeks of age.  Female Guinea Pigs can become pregnant at 3-4 weeks of age.

www.cavyspirit.com

The females are a bit more active, and tend to be more visious.