I had a dating scan at 9 weeks as I was unsure of my dates, so therefore have decided not to do the 12 wk scan as well. Ultrasounds are stressful for the baby so although they are very useful I believe they should be kept to a minimum if possible. If I was over 35 or had Downs in the family would probably reconsider but with neither of these risk factors I feel I am likely to be low risk anyway. At the 18 week scan thorough checks are carried out which can often pick up Downs as well as many other abnormaities so I would just rather wait until then and limit the stress to the baby.