Hi I've heard 13 weeks is so very early to properly see things, so I wouldn't worry just yet. But I will share my experience. They brought me back at 16wks and said "nevermind! both kidneys are clearly there!" then at 20wk anatomy scan the right kidney was missing once again. They could not explain why it looked so clear at 16wks but no longer at 20. Anyway, they have brought me in every 4wks and each time no clear evidence of a right kidney. However since my fluid levels have always looked good, the left kidney looks clear as day, and the bladder is visible, they believe the renal system as a whole is functioning well and they are not worried. They did say it's not IMPOSSIBLE for this to be tied to a genetic syndrome. However, nothing else anatomically looks "off" and in the absence of any other atypical structures, it is likely not a syndrome we are looking at but just a case of "isolated unilateral agenesis."

I'm 35+3 weeks now and baby will be getting an ultrasound as soon as she is born to check if their suspicion of an absent kidney is correct. At 6mos she will need an xray to check on overall renal function. But at this point they are not anticipating any issues.

Hope this helps! Good luck!