Ob-Gyn questions? Ask a gynecologist online.
The FDA regulates ultrasound in the United States and has cleared it for "continuous use". So you can use it as many times you want although it is better ti limit each period of time to 10 minutes.
The 10 minutes time frame is because it usually takes about 10 minutes or less to locate and listen to the fetal heart beat. Using the doppler longer than this time doe not provide any further benefit to the mother or the child. During pregnancy, it is always better to use medical devices only as far as they are beneficial.
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