Atrial Septum Defect

So... I went to the doctor today for my ultrasound and things were going well. My boyfriend and I talked with the doctor and she said everything looks fine....except for two things... 1. Our son is just under 4 pounds and I'm 33 weeks so she's a little concerned and 2. From the ultrasound she sees a faint indication of atrial septum defect and wants to do further testing. Oh my gosh I'm not happy and I'm scared. I just want my baby boy healthy...please someone put my mind and heart at ease...