Okay for anyone still reading this as just an informational post here goes...
If the girl is extremely young (5 weeks plus they are
physically able to conceive) they can be pregnant and can take care of their litter...no killing babies here!!! There might be a small litter, she may reabsorb, etc or if she's old enough you could look into an e-spay. If the ratgirl is well over 1 years old and its a first litter then there could be complications during the pregnancy and labour.
A rat goes into heat every 4 days and it only takes a second to be mated.

The pregnancy typically lasts 21-23 days, except on a post-birth impregnation (they go into heat right after birth) then the pregnancy runs up to 28 days to give the present litter a chance before they are weaned.
Some rats start to get chubby and assume the pregnant pear shape, others will fool you til a day or 2 before, and even others won't show their pregnancy til the eepers are born.
The BEST way besides a vet for telling if your girly is pregnant is to buy a digital postal scale and weigh her daily. You will see a very significant weight gain, more than a growth spurt for young females.
I had an ooops litter this morning from a shelter adoptee who was found in a backyard...I had no idea that she was exposed to a male.
Here's her weight gain chart so you can see what I am talking about
Bronwen’s Weight Gain (4 months old)
Aug. 11 – 267 g
Aug. 14 – 279 g
Aug. 15 – 289 g
Aug. 16 – 302 g
Aug. 17 – 319 g
Aug. 18 – 329 g
Aug. 19 – 346 g
Aug. 20 – 354 g
Aug. 21 – 364 g (birth)
Sept. 14 – 278 g
Before birth a lot of females start to nest, and its time to give them bedding proper for a birth then. Some rats get hormonally nippy as their instincts to protect their young kick in.