Hi there - at the 12 week scan the baby's back of neck can be measured for chromosomal abnormalities, so it can give an idea about certain diseases such as down's syndrome... but it only gives you a ratio, so a 1 in 100 chance (which is a high chance in most cases!) that the baby will have a chromosomal defect... so it isn't that accurate, but it is wise to do if you are over 35 years old because that is where they see a higher incidence and if you want to know before the baby is born then you can do it. They do measure the baby and can change your date accordingly if needed, but really it may only be a few days different unless you have irregular periods.

I had a scan done at 9 weeks as an emergency and we could see and hear the heart beating, at 12 weeks I had a doppler reading and could hear the heart rate, and could identify if there was a murmer at that time the reading was that accurate....

Sorry for such a long winded explanation but these are what I have read and/or experienced. I hope this helps!

Christy