I had open heart surgery because i was a premature infant, how does that affect me now that i'm older and pregnant?

It depends: This depends on the condition that you had as a child, how it was repaired, and the functional status of your heart today. Pregnancy is a very stressful process on the heart. This stress is at its peak during labor and continues days after pregnancy. Please see your obstetrian for appropriate cardiac studies as soon as possible.

Curious question: There is no one size fits all answer. The people that would know will be those with access to your previous history, present state of heart health, general health, etc. You may also want to consult with a geneticist on the risks your baby would have some form of heart defect.