The single most important breeding consideration is that female guinea pigs should be first bred before 7 months of age. If the first breeding is delayed beyond this time, serious (sometimes life-threatening) problems with delivery are encountered. Females should be first bred between 3 and 7 months of age. Males should be 34 months old at their first breeding. The guinea pig's heatcycle lasts 16 days. The period during which the female is receptive to the male and will allow breeding lasts about 8 hours. Female guinea pigs can come back into heat 15 hours after giving birth. This is called a "postpartum estrus," which means that they can be nursing a litter and pregnant at the same time!
Pregnancy lasts an average of 63 days. The larger the litter, the shorter the term of pregnancy and vice versa. The duration of pregnancy for guinea pigs is unusually long when compared with that of other rodents. Pregnant sows (females) exhibit a grossly enlarged abdomen during the latter stages of pregnancy. It is not uncommon for their body weight to double during pregnancy. The time of delivery may be difficult to determine because of the relatively long gestation period and because pregnant sows do not build nests. However, the week before a sow is about to deliver a litter, a slowly widening separation of the pelvis develops just in front of the external genitalia. This separation reaches slightly more than 1 inch in the hours just before delivery. This separation of the pelvis does not develop in females that are bred for the first time after 7 months of age, creating an impossible and tragic situation. Delivery of the young is not possible and a cesarean section must usually be performed to save the life of the sow and her babies. An uncomplicated delivery usually requires about 1/2 hour, with an average of 5 minutes between delivery of each baby. Litter sizes range from 1 to 6 young, with an average of 34. Litters resulting from the first breeding are usually very small. Abortions and stillbirths are common with guinea pigs throughout their breeding lives.
The young are born relatively mature. They are unusually large and fully furred, and can walk about. They also have teeth and open eyes at this time. Even though newborn guinea pigs can eat solid food and drink water from a container, they should be allowed to nurse their mother for at least 2 weeks.
Dystocia: Female guinea pigs intended for breeding must be first bred before 7 months of age. If the first breeding is delayed beyond this time, serious (sometimes life-threatening) problems with delivery are encountered. A portion of the pregnant sow's pelvis must widen for successful delivery of her young. This separation fails to develop in females bred for the first time after 7 months of age, usually necessitating a cesarean section to deliver the young and save the sow's life. Signs of dystocia include straining and uterine bleeding. Veterinary help must be sought immediately. The veterinarian will evaluate the pregnant sow by direct exam and by taking x-rays. If a vaginal delivery of the young is not possible, a cesarean section will be necessary.
Pregnancy Toxemia is a serious condition that usually occurs in overweight sows in their first or second pregnancy. Signs are most likely to be noted over 1-5 days during the last 2 weeks of pregnancy or the first week following birth. These include inappetence, depression, weakness, reluctance to move, incoordination, difficulty breathing, coma and death. Some afflicted sows may show no signs and suddenly die. There is no single cause for this condition, but stress and obesity are major predisposing factors. Others include advancing age, lack of exercise, fasting just before the onset of signs, and a large number of developing fetuses. The fundamental underlying problem appears to be inadequate blood flow to the pregnant uterus. Sows showing any of these signs must be seen immediately by a veterinarian. Because treatment is often unsuccessful, prevention of pregnancy toxemia is of paramount importance. Pregnant sows should not be allowed to become obese. Fasting and stress must be avoided, especially in the last several weeks of pregnancy. Pregnant sows must be supplied with fresh water at all times and fed a nutritious diet.