I have not had the NT scan with either of my pregnancies, I will however get it done if I have a third. When I had my 19 week scan the woman said there were 3 soft markers for down syndrome, which probably meant nothing but because I hadn't had the NT test they couldn't rule out their significance. Needless to say I was a little stressed. I didn't want to terminate a
DS baby, but noone wants there to be anything wrong with their baby.
I didn't want an amnio based on one u/s showing soft markers so after much stressing I had a second opinion scan done at another centre. Two specialists scanned me, I was there for over an hour while they analysed everything from every possible angle, and they could only find 1 of the 3 markers that were supposedly there 5 days earlier, hence I was still low risk and had been stressing for no reason. Well, I am still to have this baby so I guess we are still to find out for sure, but no amnio was recommended and I haven't worried about it since.
Even if you do not wish to terminate, the NT scan is much more accurate at detecting and ruling out problems than any later scans. Also therapy for
DS babies starts about a week after birth so knowing in advance can be a big help to your stress levels after the birth. It can also allow you to 'warn' people before the baby is born so you are not bombarded with pitying looks after the birth.
It is definitely up to you, but I would say that the NT scan is more likely to rule out problems than anything else and you can relax for the rest of the pregnancy.