Hey there!
The answer would be ultrasound.
The ultrasound is considered safe because it uses sound waves for the detection of tissue, organ, baby etc. These waves are considered safe and transfer only a small amount of heat to the tissue in which the ultrasound is done. A little amount of heat is not at all harmful to the body in any means. Early defects of baby can be detected through this procedure, the merits of the process is much higher than its demerits.
Hope this helps !