I recently had my NT scan, from my earlier dating scan I was 12 weeks and 5 days, but was measuring 13 weeks and 5 days when I had the NT scan, she said that she may not be able to do it - she said it is alot harder as the baby is more developed and moves around alot more making it much harder for them to get an accurate measurement. She did manage, but it took a long time as the baby was moving alot and was very insistent on lying on its tummy cuddled up to the placenta!