The average length of a pregnancy for a hamster is 15-21 days, so your hamster would need to have been housed with a male hamster within roughly the last three weeks in order to be pregnant. If she hasn’t had company in more than four weeks, then she cannot be pregnant. 3. Note the age of the hamster. Hamsters can start breeding from as young as 6-7 weeks of age. The only way to rule out pregnancy due to sexual immaturity of the hamster is if she is even younger than six weeks.