I'm no doctor, but I do know that one risk is that the mother could have an aortic anurism during child birth. This is of course due to the strain that is put on the heart.
A good resource is www.Marfan.Org, and of course talking to you Dr.
I have Marfan''s and have delivered 2 healthy boys naturally with a few complacations but no life threating ones. After the problems of pushing and my heart with my last son I am now getting a c-section with this pregnancy. It is scarry but well worth it. Find a dr. Who has dealt with Marfan and pregnancy and take it easy. I only have a mild case of Marfan''s though. Only my oldest has it. - best of luck- mommy with Marfan''s