Guinea Pig pregnant!?

Question
Hi there,
We bought 2 female guinea pigs for our children for Xmas.  We were told they were approx 12-15weeks of age and they were housed together with males as well.
They are in seperate cages.  One of them seems to have a VERY healthy appetite and seems to be getting quite chunky.  She does eat very well, so Im not sure if she's just gaining weight rapidly or if she is maybe PG?  Is there a way to tell?  The other one is quite small, eats ok, but nothing like the other and really hasn't grown like this one has.
It's amazing to me how different their attitudes are as well. They are both very friendly! :)  Also, the one I suspect may be PG also does this really strange hind-leg kicking when you touch the sides of her belly?!  Does that mean anyhting?
Their daily diet consists of:  Guinea pig pellets, yogurt treats, lots of fresh cabbage,carrots and broccoli, yogurt treats,and alfalfa hay.  (vitamims in their water)
Any info would be greatly appreciated!]thanks!!!!

Answer
Hello Stacie,

Here is some information I got off of a website full of Guinea Pig issues.

Breeding Considerations

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 heat cycle 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.

Here's the website:

http://www.ahc.umn.edu/rar/MNAALAS/GPig.html#Breeding

Take her to the vet for them to tell you. If she is pregnant, then she'll need more calcium and will need to eat Alfalfa hay, but the other guinea pig can't.

I hope that this helps you.

~~~Arielle