Hi again,
Yes a definative diagnosis is absolutely possible. I do have personal experience of PUV, and sadly my experience doesn't have a happy ending - however, i had extenuating circumstances, and each case is different.
My baby boy, Ciaran was diagnosed in utero at 14 weeks gestation with PUV's. He was a twin, originally a triplet but we lost one at 10 weeks, and we were told it was unlikely he would live long after birth, if his heart didn't stop before then.
We were told that PUV's can occour anytime during pregnancy, although more often it occours after birth. Essentially PUV's is a little flap of skin that grows over the uretheral opening in the bladder meaning that urine cannot be passed through the bladder to be wee'd out.
In a child that has been born it's a really simple little operation, but obviously, it's more complex in utero.
How far along are you?
We noticed that there was a potential problem at our 12 week scan and it was initally thought that one baby had an umbilical hernia, but then a follow up scan showed the PUV's. We were then sent to Cardiff's Heath hospital for a more detailed scan and they gave us the prognosis. Basicly the earlier the PUV's are seen, the worse the outcome.
The main issues are with baby's kidneys/renal functions and lungs. From about 18/20 weeks babies start to make their own amniotic fluid which is drunk and then passed through, kind of like a continuous circle. They do this to practice breathing and to help their lungs mature fully. In a PUV's baby, they cannot pass urine, so they cannot practice breathing, so their lungs can't mature.
The urine they can produce backs up into their kidneys and causes damage.
We were told that if, by some miricle, Ciaran's lungs were mature enough for him to breathe, then he would need a double kidney transplant by the time he was two.
We were sent from Cardiff to St Michaels in Bristol - is that were you went? - and we were told there was a proceedure that could be performed on Ciaran to possibly help him. It involved inserting a needle through me, into his bladder and syphoning the urine off. This though would have to then be done every five to seven days for the rest of the pregnancy. At that point i was 22 weeks pregnant. We were told that there was a 75% chance of total miscarraige every time this proceedure was done, and given that we also had another baby to think of, we felt that this wasn't a viable option for us - but it may be worth asking about for you?
My boys were born at 28weeks and four days, and Ciaran lived for just under two hours. He died peacefully in his daddys arms. We declined a postmortem as we already knew what was wrong with him.
My story doesn't have a happy ending - but that absolutely doesn't mean that yours can't!
Normal kindeys at the moment is a very good thing, for sure 
There's a Yahoo PUV's group that i used to belong to thats full of great advice and support - maybe worth a look at? Also feel free to ask any questions you may have - i'm happy to help if i can x