What is a dating scan
Instead, you will be offered another blood test between 14 and 20 weeks of pregnancy to screen for the risk of Down's syndrome.
This test is not quite as accurate as the combined test.
However this is not usually a problem, as the best time for a dating scan falls between 8 and 12 weeks, a month-long period.
Dating scans are much less reliable after 13 weeks because at this stage babies often start growing at different rates.
The dating scan is generally the first ultrasound that every mother-to-be will have.
While many women will have a good idea of when they had their last menstrual period, another portion will be far less certain.
The sonographer (the qualified professional who performs the scan) looks for key measurements to find out your baby’s age.
As medical professionals have a precise idea of the baby’s development over nine months, knowing the foetal age makes it easier to judge if everything is fine with the baby.
The combined test involves a blood test and measuring the fluid at the back of the baby's neck (nuchal translucency) with an ultrasound scan.