I just found this on the net I rememeber seeing percentage somewhere after i had had 4 miscarriagsi a row and was scared of every scan and this helped me a bit.
If the scan picks up a heartbeat and the baby appears to be the right size according to your dates, this can be very reassuring. Research has shown that if you see a heartbeat at 6 weeks of pregnancy, the chances of the pregnancy continuing are 78%. A heartbeat at 8 weeks increases the chance of a continuing pregnancy to 98% and at 10 weeks to 99.4%. So things could still go wrong, but as long as there is a heartbeat, the risk of miscarriage decreases as the weeks go by.