Nuchal scan accurate dating
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. Finding out about having twins early in pregnancy also gives you more time to prepare for the birth and for your doctor and midwife to plan your care. Nearly all scans after 10 weeks can be done through your tummy, and most units ask you to arrive with a full bladder. 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.
You may know when your LMP was, but not when you actually conceived. Ultrasound scans are considered to be safe for you and your baby when performed by trained professionals. However, sometimes, having a scan can cause unnecessary worry. 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).
For example, the NT scan for Down's syndrome has a false-positive rate of five per cent.