A little bit of liquid in babys kidney

Im 28 weeks 4 days pregnant and had an ultrasound done today. The sonogropher

called doctor in and told me tone one of my sons kidneys had liquid in it and that it can be a sign of down syndrome. Im worried because I was never tested early in my pregnancy for Down syndrome due to being moved from doctor to doctor for having a low lying placenta in the beginning. Anyone else have been told about this ? Please share your experience with me first time mom as well