She keeps her young with her for a year, and in the intervening season she does not breed. The growth rings on her teeth will be thin during the year she is pregnant because all her calcium goes to the fetuses. In the year she raises them, the ring will be thick. Bear litters can be from one to five cubs. The average is three.
Breeding takes place in the summer, but the embryos go dormant.
Pennsylvania bears don't hibernate as western bears do. Their body temperature remains normal and though they doze for many hours they can be roused to alertness easily. It's their metabolism that slows, and along with their greatly reduced physical activity, they regulate the consumption of the body fat they've accumulated over the summer. I saw a bear fetus taken from a road-killed mother who didn't make it to the den that year. It was less than two inches long, a deep chocolate color, and it looked like a plastic charm for a bracelet floating in a jar of preservative.