That whole thread in the first trimester about the ultrasound is just making a mountain out of a molehill and people throwing out articles found online and causing unnecessary concern. I mean logically thinking, do you really think that they would have ultrasounds, made for viewing something as sensitive as a fetus, to give off such high frequency waves that it would terminate the pregnancy or alter the DNA? I mean these things are literally made for the exact purpose of checking on the baby. After all these years they have been around, you know it's been tested on animals. High frequency waves is not something new to science, there has been so much research on high frequencies, it's almost common sense to only use low frequency waves to view a baby.
IMO, that thread was a ticking time bomb. It caused women who are already nervous as hell about being pregnant to panic more than they already do. If you're against ultrasounds, that's fine. But there's no need to throw out "facts" that are just not there. You'd have to have a hell of a lot of ultrasounds, for extended periods of time to alter anything. XRays are far far more damaging than ultrasounds ever will be, and there are people who have to have XRays regularly for medical purposes and their DNA doesn't get altered. To me, it's more dangerous for a pregnant woman to be driving a car than it is for her to get an ultrasound. We take unnecessary risks every day being pregnant, having an ultrasound isn't going to do anything.
Sorry for the rant. I'm very much a facts person, I don't believe things until I see solid proof. And when I see these poor women believing everything they read on google it just drives me crazy. Using your doppler will not harm your baby. Unless you're using it for hours every day, you shouldn't notice any difference. It's the extended exposure to these types of things that are dangerous, not the general usage.