I often visit the University of Maryland Medical School's website on herbs. It has a TON of info including studies, Rx interactions and contraindications.
St John's wort has a large write up and includes details that it's appropriate for mild to moderate depression or those with seasonal affective disorder. It is not good for severe depression. Do not take if you are bi polar.
Also it can interfere with birth control pills, which I have heard from other sources.
Website says it is not recommended for those who are pregnant or nursing. It does not say why. It could be that it's never been tested on anyone pregnant or nursing so they just don't know.
Details here: http://www.umm.edu/altmed/articles/st-johns-000276.htm