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. The dating scan will also show if you're expecting twins, triplets, or more.
The sonographer will put some gel on your tummy and move a small hand-held device (transducer) over your skin to get views of your baby.
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.
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).
It won't hurt, although you may feel a little pressure on your tummy.