Ultrasound revealed TWINS!!!!

I am 7 weeks and 5 days. I had my first ultrasound yesterday. We were so happy when my Dr told us we are having twins. Then she told us not to get to excited. 1 baby is measuring perfect with a heart rate of 135!! But the other is smaller and measuring a week behind with a heart rate of 95 (she said it should be at least 100). She said there is a 50/50 chance that baby won't survive. I will have another ultrasound in about 10 days to check progress. Has anyone had a similar experience or know someone who has???