Hi Bel,
I have had NT with all of my babies except Ruby my oldest daughter who is 10.

I go to a specialist ob a foetal medicine obstetrician. He is a bit of a legend up here and over the years I have used him for my nt and 2 amnios and he has done all of my morphology scans at 18-19 weeks.

This is what his practice state on nt ultrasounds:

The blood test is required 3 days prior to the ultrasound and is best done in the morning. The reason for this is that the test is measuring hormone levels and the most accurate readings are given in the morning. HCG levels are often high in women who have babies with a chromosomal disorder but not necessarily that is why the blood tests alone are not an accurate indicator of a problem.

The nuchal fold is a fluid filled sack at the back of your babies neck. All babies have this but they don't stay for long that is why the scan needs to be done roughly between 11 weeks and 5 days and 13 weeks and 5 days. This practice prefers to see women at 12 weeks and 3 days. After 13 and a half weeks the babies growth makes the measurement impossible. Babies with a chromosomal disorder often but not always have a larger than expected nuchal fold - again this is why the scan isn't as accurate done without the bloodwork.

The results of the nuchal measurement combined with the blood results are calculated and a risk factor is given. For example a woman may have a risk of say 1:200 of having a baby with a chromosomal disorder just calculated on age alone. After a favourable test the risk factor may come out at say 1:1500 which are very good odds that all is good.

This is a screening test and is by no means definitive. But, I believe that this test picks up a very high percentage of pregnancies with a problem.

Bel, personally I highly recommend the nt scan. It can really put your mind at ease. Each practice is different so I would just follow the instructions given by the practice you are going to.

I wish you lots of luck and I hope this has helped a bit.
Love Deb