An ultrasound is a technology that uses high pitched sound waves that bounce off from the baby and are collected by the transducer (the device that the technician glides over your abdomen). These sound waves form a series of images on the monitor.
A normal abdominal ultrasound, if properly done by a trained technician, produces a rise in temperature of about 1 degree Celsius in the fetal tissue. With time, this temperature subsides back to normal. Scientists and researchers have reported that only a rise of 4 degree Celsius or above may harm a baby. For this to happen, a transducer will have to be placed on one location for about two hours!
A trans-vaginal ultrasound produces a little more amount of heat but still not enough to cause any harm to the fetus.
A 3-D ultrasound is equally safe but a 4-D ultrasound is not advised during early pregnancy as it is unnecessary and produces more heat than other ultrasounds.
Long-term effects of ultrasounds are still being studied but there is no evidence to suggest that ultrasound causes any harm to the baby.
As opposed to X-ray, no ionizing radiations are used.
However, it is best to have ultrasounds done by a trained technician only when indicated.
Read more: 3D/4D Ultrasound During Pregnancy