Other aspects that can affect the accurate calculation of the baby's age include:
- The operator's qualifications, skill and experience at performing and interpreting pregnancy ultrasounds.
- Measuring the 'gestational sac' volume during very early pregnancy (less than 6 to 7 weeks), which is less accurate than measuring the baby's 'crown to rump' length.
- The time taken to perform the scan. Faster examinations tend to be less accurate than if the operator takes time to obtain precise measurements.
- The type of equipment used (digital machinery is better then analog) as well as the software used to estimate dating statistics.
Genetic testing
Genetic tests are used to screen for and detect specific inherited disorders in unborn babies. Many of the genetic tests available involve the use of ultrasound. These can include:
A nuchal translucency or 'NT'.
A nuchal translucency uses ultrasound to visualise and measure a fluid filled sac at the back of the unborn baby's neck during early pregnancy. NT is usually performed between 11 weeks + 3 days and 13 weeks + 6 days to estimate if the baby is at increased risk of having a chromosomal abnormality such as Down syndrome (also called 'trisomy 21'). You can read more in nuchal translucency.
A 'chorionic villus sampling' (or CVS).
A CVS takes a small sample of cells from the baby's placenta and examines them to 'map' the baby's genes or chromosomes with the aim of identifying the presence of a genetic disorder (such as Down syndrome). Ultrasounds guide the caregiver so that they place the needle in the correct location and avoid the baby. A CVS is usually performed between 10 and 12 weeks of pregnancy. You can read more in CVS.
An amniocentesis (or 'amnio').