Baby boy kidney question

I went to the doctor Friday at 19w5d & my doctor noticed that my sons left kidney had a little less fluid than the right. He said its very common in boys & it normally goes away by 28 weeks or by birth. But then he had me have blood drawn to check for Down syndrome & other things. He said if it comes back negative that he definitely won't be worried about his kidney but if the test comes back positive I'll have to see a specialist. He said to still not worry if it comes back positive because it is common in boys. Has anyone else gone through this? He said not to worry but I'm so worried!!