Rats reach sexual maturity at about 4 to 5 weeks of age. From this age onward, females and males should be housed separately. The average gestation time is 21 to 23 days for rats, and pregnancy is sometimes detectable at about 2 weeks by feeling the abdomen or noticing weight gain or mammary (breast) development. Pregnant females will make a nest, and they should be provided with suitable materials. Tissue paper provides excellent material for nesting.

The usual litter size is 6 to 12 pups. When baby rats are born, they are deaf and blind. The cage should be kept in a quiet place and the litter should not be disturbed for at least 7 days after birth, especially if this is the female's first litter. Weaning occurs about 21 days after birth. Female rats can quickly become pregnant again after giving birth; however, it is not healthy for a female rat to be both pregnant and nursing a litter. It is recommended that the female be given a rest period of at least 2 months between pregnancies and litter rearing, to restore her body to full strength.

Breeding and reproduction in rats can decrease due to various factors such as age, malnutrition, abnormal light cycles, cold environment, cysts on the ovaries, tumors, and inadequate nesting material. Pregnant females may abort, abandon, or eat their babies due to inadequate food or lack of water, overcrowding in group housing, inadequate nesting materials, sick or deformed pups, or excessive noise. In healthy rats, however, reproductive problems are uncommon.

Last full review/revision July 2011 by Katherine E. Quesenberry, DVM, MPH, DABVP (Avian); Kenneth R. Boschert, DVM, DACLAM