does prometrium cause a positive result on a pregnancy test?

Answer 1

No.

Prometrium is a "progesterone", a sex hormone similar to natural hormones produced by a non-pregnant woman.

Pregnancy tests test for hGC, a chemical made by pregnant women, but NOT by non-pregnant women.

Here is more info on hCG and a list of possible causes of a falsely positive pregnancy test:
http://en.wikipedia.org/wiki/Pregnancy_test