Pregnancy with a Congenital Heart Condition

Ok, so I'll preface this by saying I'm not planning on getting pregnant for a long time, but I'm curious of the effects on my heart if I do. I was born with pulmonic stenosis and had a new valve implanted in me about 3 years ago.

I recently set an appointment with my cardiologist and kind of off-handedly asked about getting pregnant. He said that I could probably have a safe pregnancy, but there would have to be a lot of monitoring to ensure that everything would be ok. Can anyone explain what will need to be monitored in a scientific way, and why? Is it because my heart could get bigger and screw up my valve somehow, or is it just because of the extra blood pumping through it? Obviously this isn't a super serious question for right now, I'm just looking towards future possibilities.