Ultrasound has been around for over 30 years. To date no risk has been found related to diagnostic ultrasound. A few years ago the FDA regulated the amount of energy that could be used on fetal ultrasound. All machines available today are all factory set to these levels.
Web sites of Interest:
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Many studies have been published that study the effects of ultrasound on the fetus, a few of these are:
Effects of repeated prenatal ultrasound examinations on childhood outcome up to 8 years of age
Effects of Ultrasound on maternal-fetal bonding: 2D vs. 3D