Amy my OB told me that this is exactly what can happen at 12 weeks. the doppler is only so good. He has told me he will not do them at 12 weeks as it only stresses the mum out if it is hard to find, only 16 weeks on. Then the heart beat is much easier to pick up. Please try not to worry too much. I know easier said than done. I am sure that all will be fine.