It takes about 21 days for a rat to have babies, and if she's noticeably getting bigger leading up to 21 days since the last time you noticed mating, then you definitely have pregnant rats.



Mice have a slightly shorter gestation period, 19-21 days is the usual range.



Another way to know your rats are pregnant is if all breeding stops. A rat's cycle is once every 5 days and they go into cycle (or heat if you will) within 24 hours of given birth as well. So if you stop seeing any signs of mating after 5 days since the last sign of mating then you know she's pregnant.



Mice I think have a slightly shorter cycle, I think its 3-4 days but not 100% sure on this part.



Keep in mind though that if environmental conditions aren't right for rearing babies, both mice and rats can put a 'pause' on their pregnancy till conditions are ideal. Thus if food quality is poor or insufficient or temperatures are not right you will notice no babies will come. Overcrowding and stress will cause this too and mice who do have babies anyway, tend to eat them up if conditions aren't right.



A new colony of rats can take up to 3 months before they have their first litter even with animals who are at the best age to start breeding when they are all put together. They seem to need this settling in period before babies start appearing.
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PHLdyPayne

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