Male guinea pigs reach sexual maturity, when they are about 4 to 5 weeks old, while the females reach sexual maturity, when they are about 3 weeks old. They are one of the most fertile species and are able to breed all through the year, however, it is in spring that birthing peaks. During one pregnancy, a female may give birth to as many as five litters. The gestation period in guinea pigs is anywhere between 59 to 72 days, while the average is around 63 to 68 days.