My daughter has three partially functioning kidneys -- you would think that would be better than two but it isn't. I am not aware of anything that they do in utero for kidney problems, my daughter is now 16 and this wasn't detected until she was born.
The first thing they will likely want to do after she is born and confirm the function of the "good" kidney is to sort out whether the "only one kidney" is an isolated issue, or if it is part of a larger syndrome. Sometimes this will be obvious from other issues that may be visible or easily detectable when she is born, sometimes it won't be. Kidney problems are frequently associated with hearing loss, and can also be associated with issues with the spine, jaw, heart, and balance. A full work-up will probably include back and neck xrays, a CT, and an ABR (which is a sedated hearing test for infants) if the newborn hearing screening turns up anything wonky. If she has any these issues in addition to the kidney problem then you will likely want a full craniofacial team assessment if the ears/jaw are involved as well as a genetics workup.
You can't tell my 16 year old from any other sixteen year old by just looking at her. With some assistance (she is hearing impaired) she does extremely well as a junior in a mainstreamed school. She is on a low-sodium/low animal protein diet. They told me when she was born she might be "short" and she's 5' 8" == granted in my family that might BE short, as I and her older sister are 5' 11" In her case, the kidney issues were the result of a syndrome, so she's had ear surgery, jaw surgery, and back surgery and come through all of them with flying colours, the kidney issues we only monitor at this point. As a point of reference, her unrelated grandaunt lost a kidney in a car accident almost 55 years ago and has never had any major health problems as a result of that.
Good luck and try not to stress.
S.