How Old Does A Guinea Pig Have To Be To Get Pregnant?

From 4 weeks old  but remember that at 4 weeks its still a baby itself  around 5 months is about right if you were going to mate any later and her bones can fuse and she will not be able to deliver the babies and would then die!
I am assuming you are wanting babies please if you really have to do this and can't find it in you to rescues the ones that don't have homes and are stuck in tiny hutches waiting to be loved (guilt trip over)
then read EVERYTHING you can get your hands on books internet everything make notes because you wont remember or you ll forget the site you were on .
I spent 6 months every night reading about guinea pigs when the kids were in bed
also my piggies belong to me I am responsible for them I never let the children think they belong to them they have to ask before playing feeding or cleaning them out
I know they are very cute and so loving as long as you spend lots of time holding grooming etc
but they do take up a lot of time and what would you do with the babies?
Have you room to keep them ?
Please I beg you read LOTS and then decide    if you suspect the worst has happened and shes already pregnant then start reading NOW make sure she has vitimin c lots of fruit and veg plenty of water as she will be drinking more and a vets number at hand in case you need one

well good luck

An easier and short vesion is youll know because it will start to try and mate with its mother or anyother females in the enclosure .plus it will make a noise like vibrating