There is only a small window of opportunity (about a 2w period) in which to have the nuchal fold scan. It is usually done between 11 - 13w, most commonly at about 12w. The reason it can be done between 11 - 13w is because each baby's growth rate is so different, some people are able to successfully have the scan done earlier/later. It is not possible to have the scan done any later than 13w though. A blood test is normally done in conjunction with the scan to more accurately determine the chances of downs syndrome.

The other 'major' scan is usually done between 18 - 20w where all the organs are checked (and where people may find out the sex if they wish, and provided a clear image of the bits and pieces is available!).

HTH!!!