Ultrasounds are generally regarded as safer than x-rays because they don't use ionizing radiation, according to the U.S. Food and Drug Administration. However, while no known health risks are in play as of 2014, the FDA reports concerns about tissue heating and fetal exposure to multiple ultrasounds.
WebMD also notes that ultrasounds aren't viewed as hazardous. However, the FDA points out that ultrasounds heat the body and sometimes cause gas pockets in tissues or body fluids. The implications to a woman's body aren't known as of 2014. The FDA also states that some U.S. and international organizations encourage only essential use of ultrasounds based on uncertainty of risks to fetuses.