Guinea pigs are sexually mature by the time they are 4 to 8 weeks old and can produce up to four litters per year. It's not uncommon for a young female guinea pig purchased at a pet store to be pregnant when you bring her home. While the only sure way to tell how far along your guinea girl is in her pregnancy is by a veterinary exam, there's a few clues that can help you determine her timing.
Watching Her Figure
A common guinea pig move is to walk forward with the front legs, leaving the rear legs in place. When your guinea is stretched out in this fashion, a glance at her sides will help you know where in her 8 to 10 week pregnancy she may be. A nonpregnant or newly pregnant guinea pig's sides will be straight when she is stretched out. By mid-pregnancy, she looks like she has a tennis ball inside her. When she is ready to give birth, she will look like she swallowed a softball and her hips and hind legs will be wide to accommodate the impending birth.
Watching Her Weight
An average guinea pig weighs about 26 ounces before she becomes pregnant. If she has given birth before, she likely will double her weight by the time she gives birth, so you can approximate how far along she is by weighing her. A first-timer will not gain as much weight, but generally will weigh 35 ounces or more by the time she gives birth.
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