It doesn't make logical sense that St. John's Wort would be unsafe while Prozac is considered safe. All it really means is that Prozac (and others SSRIs) have been extensively studied while St. John's Wort has not been studied by modern medical researchers as much.
If you've done well with St. John's Wort in the past, while Rx antidepressants haven't worked well for you, then I'd go with what works. I'd put that in the same category as "Choose the drug that works best for you, even if it's not the one proven safest for the baby" as per Dr. Hale- his attitude is that so little med gets to the baby that the choice of med makes very little difference to the baby, having a healthy mom is the primary concern.
So, even if the Rx meds are "safer" than St. John's Wort, the herb can still be a better choice if it's the more effective medicine for you.