Hello, I would be happy to help you with your question.
From what you are saying, your baby most likely has a small choroid plexus cyst
- these can be seen as an isolated finding and lead to an "enlarged" gland. To me, it sounds like they did not explain it that well, and it might be worth seeking clarification on exactly what they saw ("was there a cysts in the gland?"). As an isolated finding, and in the setting of normal first trimester screening AND with a Perinatologist telling you "not to worry", then it seems like the risk of amnio is not worth taking. Nonetheless, if you are worrying, then you deserve a re-explanation of what is going on. These are usually gone by about 28 weeks, so you could also ask for the followup scan.
I hope that this helps and good luck!