Early pregnancy dating ultrasound
The scan will usually show that your pregnancy is less far along than suggested by your LMP.
This means that accurate dating reduces your chance of having your labour induced because your baby is thought to be overdue.
Most units offer a screening test for Down's syndrome and other conditions at the same time as the dating scan.
Screening for Down's syndrome involves a nuchal translucency (NT) scan plus a blood test. You can have the combined screening test from 11 weeks to 13 weeks plus six days, or when your baby's CRL is 45mm (1.8in) to 84mm (3.3in).
If you do know the conception date, this is counted as day 14, not day one.
However, sometimes, having a scan can cause unnecessary worry.
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.