Congratulations on your baby girl!
My son is 12 years old and we found out in utero that he would probably be born with Down syndrome. It has been an absolutely wonderful, enlightening, and positively endearing 12 years with him! Parenting him has been full of joy and giggles and he has taught us more about the beauty of life and love far more than we could ever teach him.
There are times when it will be difficult, when reality sets in and you realize that from time to time your daughter will be discriminated against, or her struggles will seem insurmountable, but if you approach it with a positive attitude, then others will follow your lead and the road is a lot less rocky. And, as with any child, as parents, you set the stage for the direction in life your child will follow, so never put expectations or limitations on her and accept her for who she is. You'll find out she will boggle your mind with her accomplishments and that for every time you hear the words, "She can't", you'll turn with a smile on your face and say, "Just watch her go!" And she will.
I'm unfamiliar with Endocardial Cushion Defect directly, but I think its also known as Atrioventricular Septal Defect (AVSD) and any Down syndrome message board will lead you to parents who have dealt with this issue and can help you out with questions.
This site is run by a doctor who has a son with Down syndrome. I usually start here when I need to research something and then follow the numerous links to medical issues and message boards from that site.
http://www.ds-health.com/
Karen