Breeding Facts... The best time to breed a female for the first time is at 4-5 months of age. Female rats do not have periods but come into heat about every 4 days. A domestic female rat can become pregnant by the age of five weeks. An average litter size is 10-12 kittens. The gestation period is normally 22 days. Rats open their eyes at fourteen days. Rats can be completely weaned by the age of four weeks. By 5 weeks of age, baby rats should be separated by sex. A domestic female rat should not be bred more than 3-4 times in her life time.