they think it is psychosomatic
I've had psychosomatic itchiness throughout my current pregnancy, I am still battling it.
It started early in the pregnancy and is all over, although most often my back, hands and feet. I've had all the tests and bloods. I accept that mine is psychosomatic because I had a highly anxious early pregnancy.
Do not underestimate the power of the mind in making the itch very, very real.
The bad days were horrible. I honestly sometimes wished I could scratch the baby out of me and imagined seeing myself all bloody and butchered, but perversely relieved. I would also imagine my baby in the same distress as me but unable to itch. It made me want to end the pregnancy to end the suffering 
I have had to do some self-psychology (your Mum could see a specialist for this if she cannot manage it herself). This began by fully accepting that it was in my mind and then developing strategies to distract myself.
I solved the issue by 2 weeks of utter exhaustion. I did not allow my brain to rest once, not at all. I moved from one task to the next. As soon as I started thinking about the itch I would change tasks and focus diligently with all my mind on something else - reading, crosswords, mumsnet (it was the reason I found this site), Sudoku, Candy Crush - anything.
The main thing was to not stop, not for a second. I became very tearful because I was so exhausted by it all. Sleeping for the first few days meant that I had to read and read until I literally passed out mid-sentence in a book. If I stopped reading while still awake (even if eyelids were drooping) then the itching thoughts came back. So I had to read and fall asleep with kindle still in hand.
I also had to stop those ideal few minutes in the morning when you wake up and have a few minutes to come around before properly waking. I had to immediately start doing something to distract from my itching thoughts.
Eventually, this relentless activity got me so, so very tired that my sleeping stopped being disrupted and I would get to sleep sooner and sleep for longer.
The improved sleep helped my daytime moods more and slowly, gradually, I didn't need to distract quite so much. I could rest my mind a bit more each day. This incidentally coincided with reaching the point in my pregnancy when I felt less anxious (23 ish weeks, when I started to consistently feel the baby move which was the reassurance I needed).
So maybe your Mum could team the distraction techniques with some stress and anxiousness reducing techniques.
I feel for your Mum, I know how all-consuming a non-medical itch is. If all tests prove negative then sleeping tablets and/or anxiety medication would probably help her. That or a very determined mind-set to overcome the silly games out mind plays on us when anxious.
Best of luck to your Mum. And sorry about the long post!