I found a discussion on doppler versus ultrasound, here on Mothering! Mothering Doppler or Sonogram thread And apparently Doppler is the highest acoustic output technology currently in use, and must be used with caution, and as little as possible. But we must also remember that the majority of evidence to the safety of ultrasound, was based on epidemiology gathered on people scanned before 1992, when allowable limits on energy intensity of ultrasounds, were raised considerably, so what was safe for people then, may not have remained safe once the ultrasounds became several times stronger, after! But is Doppler still more dangerous?
Of concern is this I found on Doppler: "Among all ultrasound modalities, spectral (pulsed) Doppler is the one with the highest acoustic output and, thus, the one with the greatest potential for bioeffects 58. A recent animal study seems to justify this concern 59. Chicken eggs were insonated by B-mode or pulsed Doppler on day 19 of a 21 day incubation period. Exposure was to B-mode for 5 or 10 minutes or to pulsed Doppler for 1 to 5 minutes. Eggs were allowed to hatch and learning and memory tests were performed in the chicks on day 2. Impairment in ability to learn or in short, medium and long-term memory was absent after B-mode exposure but was clearly demonstrated for those exposed to Doppler, with a dose-effect relationship. Furthermore, the chicks were still unable to learn with a second training session 5 min after completion of the initial testing. This represents a dose-effect relationship with higher time exposure to Doppler associated with clear behavioral effects." here's the link to that article.
Also in that article was this:
"There have been several major reviews published of epidemiological studies conducted over more than 25 years 53-57 . Some of the studies have serious limitations such as small samples, poorly matched controls and, perhaps more importantly from a bioeffects standpoint, absence of information on acoustic output and quantification of exposure (number of episodes, duration or “dose” delivered to particular target). This is particularly relevant in today’s clinical situations because of the addition of new modalities, with potentially very high energy levels (such as spectral Doppler) and the expansion of diagnostic studies to the first trimester, which is known to be a period of high sensitivity of the fetus to teratological insults (see above). In addition, it must once more be stressed that there has been no epidemiological study published on populations scanned after 1992, when regulations were altered and acoustic output of diagnostic instruments permitted to reach levels many times higher than previously allowed (from 94 to 720 mW/cm2 ISPTA for fetal applications)."
Just some things to research and consider, in order to make informed decisions. :)