Thanks for your query.
Normally, the nuchal thickness is measured at around 12 - 13 weeks.
18 and 20 weeks is slightly late for measuring it.
However, your value is definitely higher than normal.
You should see a genetic counsellor anyway, as you can go for further invasive testing if your pedigree and detailed evaluation so mandates.
If you are 28, and reasonably healthy, with no positive family history, there is no cause for undue stress
, as it will impact your pregnancy
Also, you can consider further testing in the form of triple marker etc.
Do consult a genetic counsellor.