There are not any studies about this drug and pg but there are many about prescription drugs that are effective at helping depression and are safe if you get pg. There are several girls on the Playgroup board who took anti-depressents during pg so maybe post there? Your RE will not decide you can't get pg; believe me they are here for the $$.
Here is a link about St. Johnswart and pg:
http://parenting.ivillage.com/pregnancy/pcomplications/0,,midwife_3q0k,00.html
I know that you cannot go out in the sun if you take this drug b/c it becomes toxic when exposed to light (and the chemical is all over your body) so things such as skin rashes can result. Link:
http://health.yahoo.com/skinconditions-medications/st-john-s-wort/healthwise--d04408a1.html
I would not take it if it were me; just my honest opinion.