Ultrasound is:

non-invasive (the patient does not have to be cut open or have anything inserted into him/her) of monitoring foetal development using ultrasonic acoustic waves.

non-ionizing. It does not employ ionizing radiation, which would be dangerous foetus, as radiation risk is greatest when much cell division occurs, and disruption of DNA would be most harmful when organs are developing.

Ultrasound is considered safe. The AIUM (American Institute of Ultrasound in Medicine) in 1977 announced that "no independently confirmed significant biological effects have been demonstrated in mammals" exposed to acoustic intensities of less than 100 mW cm² - over the last twenty five years collected data reveals that this is a conservative upper limit. (see adverse reactions).

relatively cheap! This does not affect the patient's health - but rather the health of the hospital's bank balance.