Have you considered the following:

Article Ultrasonographic Diagnosis of Pregnancy in Rats

Dear Mr Wielders,

Thank you for your answer. It looks like a good method, however not accessible for me at the facilities where I work. I was hoping for some straightforward biochemical test (maybe through urine?) that would be feasible in our laboratories. Nevertheless, thank you for your input.

Best regards,

Rona

Dear colleague,

Of course you could try to collect some urine and determine progesteron. I am not sure if a kit for human use would serve, but you can give it a try.

Google rat pregnancy hormone and you will find things like

http://www.flyfishingdevon.co.uk/salmon/year2/hormonelevelsinpregnancy/hormonelevelspregnancy04.html

good luck