The gestation period of mice is normally 22 days but it may vary from 21 even up to 25 days. They will then reach sexual maturity at the age of 5 weeks. Matured female rats will heat every 4 to 5 days, all year round. The pregnant rat will show some nest making if she is about to give birth, usually 2 to 3 days before it?s schedule. A mother rat will heat again after giving birth so it is important to separate a male rat to avoid the mother rat from getting pregnant again.