How long are guinea pigs fertile?

As to estrus - The time prior to estrus, called proestrus, lasts about 1� days. During this time, the sow shows increased activity and mounting behavior. The fertile period, called estrus, lasts 1-18 hours with spontaneous ovulation occurring about 10 hours after onset of estrus. As far as what age do they go into a guinea pig type of menopause? I don't think they do. From what I can tell, they stay fertile to the end. As you already know, the only way to ensure your older piggy does not have an unwanted pregnancy is to neuter or spay or to maintain celibacy.