It shouldn't be uncomfortable, and you don't need a full bladder Your sonographer will give you a report of the scan, which will tell you exactly how many weeks pregnant you are. Keep it with your maternity notes so your midwife and anyone else involved in your care can see it. This measurement, known as the crown rump length (CRL), is very accurate in the first trimester. The point of the scan is to work out how many weeks pregnant you are. Ectopic pregnancy and miscarriage: diagnosis and initial management CG 154. You may have calculated the length of your pregnancy from the first day of your last menstrual period (LMP). As long as the timings are right, you can have the NT scan and blood test at the same appointment as your dating scan. That's because the screening test needs an accurate due date to be reliable. Hormones in your blood vary according to your stage of pregnancy. The scan will usually show that your pregnancy is less far along than suggested by your LMP.
You could have a dating scan without NT screening, or even change your mind on the day of your scan. If you do know the conception date, this is counted as day 14, not day one.