Our baby boy has this issue and it is being monitored with ultrasounds every 6 weeks until birth and then he will be seen by a pediatric urologist after the birth and for as long as needed to make sure it isn't causing any issues for him.
Our perinatologist told me that it is a condition they see far more often in baby boys than baby girls and that seeing it in a girl is more cause for concern for them. He said it is usually caused by a narrowing of the ureters which causes waste to flow too slowly or back up into the kidneys which causes the dilation. He said that 90% of babies show full improvement by birth and that of the rest, 90% show full improvement by the first year. A small number of children will require surgery to correct the condition if it is causing issues.
Also, our perinatologist said that his daughter was born with the same condition and was monitored for several years. Her condition never got better but it never got worse either and since it never caused her any issues they eventually stopped monitoring it and chalked it up to "just the way she was" so just because your baby might be a girl it doesn't necessarily mean much more than it would for a boy.
I go for my next ultrasound on 12/14 and then I will find out if the condition is the same, worse, or better. The doctor said that as long as it is stable or better then that is a good sign that it is not serious. He also told me not to worry myself until I actually had something to worry about which is advice that I am taking to heart.
I hope this post helped you! Just remember that the vast majority of these cases work themselves out on their own.
ETA: He also said that babies with kidney dilation that is still present at birth may be put on an antibiotic regimen because the backing up of waste makes their risk of kidney infections greater.