The Hamster Estrus Cycle

I am doing an assignment in college about hamster reproduction. I was wandering if you could tell me about the signs of pregnancy in hamsters and about the estrous cycle or point me in the right direction to find this information.

The females come into season for at least 4 hours every 4 days throughout the year, and immediately after giving birth. Being nocturnal, they are likely to be receptive to a male only at night. Even experienced breeders don’t know a definitive way of telling when a female is in season. Only a hamster in good health is capable of bearing a large litter and they quickly become exhausted by having many litters. No hamster should be expected to have more than 3-4 litters in her life.