There is no sensation at all from the ultrasound waves.
A short history of the development of ultrasound in pregnancy can be found in the History pages.
If this is observed, the probability of a continued pregnancy is better than 95 percent.
Normal heart rate at 6 weeks is around 90-110 beats per minute (bpm) and at 9 weeks is 140-170 bpm.
At 5-8 weeks a bradycardia (less than 90 bpm) is associated with a high risk of miscarriage.
For example, if ultrasound scan demonstrates a 7mm embryo but cannot demonstrable a clearcut heartbeat, a missed abortion may be diagnosed.
In such cases, it is reasonable to repeat the ultrasound scan in 7-10 days to avoid any error.