Hi,
I have been treating pregnancy-related problems in close association with OB-GYNs for over a decade.
Firstly, i wouldn't worry too much - i personally haven't heard of any problems associated with treatment with Ultrasound or interferential. My understanding is that the risks are theoretical but we are talking about precious life here so why risk it, right?
Secondly, if you have moderate to severe low back pain, then i would consider TENS as an alternative for pain modulation. To purchase a unit is relatively inexpensive and a lot of women use it during labour. It is safe for use during pregnancy becuase the current doesn't dive deeply like interferential does.
Ultrasound is likely to be more damaging to the baby in earlier stages of development, when things are still rapidly developing. You have to remember that going via the back, there are at least 2 inches of tissue/muscle/bone to get through before you get to the baby. I would only really be concerned (IMO) if he did U/S directly over your tummy.
Interferential is also likely to be a problem (again IMO) if the electrodes are placed so they go through your abdominal cavity. If the electrodes are ALL placed on you back, then it is unlikely that the current is passing through your baby. The reason why is because the electricity seeks the easiest path to the other electrode - and that is via muscle which is very water/blood rich.
Lastly, though the pain may be bad, there are other physical treatments that may help. We can't 'fix' everyone but we can usually provide some measure of relief - and let's face it, a little relief is better than nothing!
If you feel REALLY unsure about what has happened, then you can always check on your baby via a diagnostic ultrasound. Costs money but the peace of mind for you may be worth it...
I hope that has allayed some of your fears and concerns. I am not advocating for the chiropractor, simply just trying to present some information...
Best of luck with having your baby. I hope all is fine - i think there are many more risks coming your way that are more dangerous than U/S and interferential (birth!!).
Cheers