the only thing that can mess you up is if she is pregnant
Thats incorrect. Labs are not allowed perform such tests on urine samples collected for a drug test.
Even if a lab were to break the law and ran such a test, the lab would not report that the male donor is "pregnant". The lab would state that the male donor has prostate cancer or testicular cancer.
Just as HCG horomones are elevated in expectant mothers, they are also elevated in males with testicular cancer, or prostate cancer.

The "pregnant" story is nothing but a myth taken from an old Cheech & Chong comedy skit.