If you are overweight or your body tissue is dense, sometimes this can reduce the quality of the image because there is more tissue for the ultrasound waves to get through before they reach the baby.
For many women, ultrasound scans are the highlight of pregnancy.
The sonographer estimates when your baby is due (the estimated date of delivery, or EDD) based on the baby's measurements.
The dating scan can include a nuchal translucency (NT) scan, which is part of the combined screening test for Down's syndrome, if you choose to have this screening.
For example, you may be offered further tests, such as amniocentesis, that have a risk of miscarriage.
The first scan is sometimes called the dating scan.