It's entirely up to you whether or not you have a scan.
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). That will happen at your next routine scan at about 20 weeks However, some major problems can be seen at your dating scan, such as issues with the umbilical cord or with your baby's skull or abdominal wall. You may need a second scan to check the first scan's findings. It may also help you to avoid feeling disappointed towards the end of your pregnancy if you think your baby is late when he's not.
The scan can check that your baby's heart is beating nicely and that he's developing normally. It may show a very minor problem, or something that may get better on its own. onlinelibrary.[Accessed July 2017] Loughna P, Chitty L, Evans T et al. Fetal size and dating: charts recommended for clinical obstetric practice. As with all screening tests, there can be false-positive and false-negative results. Implementation of maternal blood cell-free DNA testing in early screening for aneuploidies. For example, the NT scan for Down's syndrome has a false-positive rate of five per cent. This means that, out of 20 women who have the scan, one woman will be wrongly told she has a high chance of having a baby with Down's syndrome. The scan will usually show that your pregnancy is less far along than suggested by your LMP.