My babies neck tissue measurement was high and they say he could have down syndrome. Can you explain this?

Need more info. Babies with down syndrome typically have short necks with extra tissue. Having this noted on ultrasound does not necessarily mean the baby has down syndrome, but other tests will be necessary to know this.
Screening tests. Screening tests are chosen to look for possible issues in a way that not too invasive (or expensive) but can narrow down the group of subjects that need closer scrutiny.After studying thousands of fetal exams. they have normal ranges for features. DS kids may not fit in with these norms, so if the study is suggestive, additional testing is recommended.