Look at the bulging sides. They aren't heavily preggo yet but they are getting there. Female rats go from a fairly sleek look to a definite pear shape as they approach the final week of their pregnancy. Some very big bodied adult female rats can hide a pregnancy but not if it is a good sized litter in there.
Yep like all mammals the bulge of pregnancy is seen in the lower abdominal area.
Here's some photo's to explain....
This female rat was at this time about a week away from delivery...
About a week later she's definitely heavy with her babies. You can see how "thick" looking she is as she waits in her nest to deliver....
Same female as above the day she delivered, very swollen with babies. She was a very big bodied female and carrying a very good sized litter of 18 babies in there. This picture was the day she delivered that litters (all the babies survived and were in fact quite large at birth). She was so big by this point that she could no longer even rise on her back legs to greet me.....