12 week dating scan accuracy
Charts have been developed for this purpose, but some simple rules of thumb can also be effectively used.From 6 to 11 weeks gestational age, the fetal CRL grows at a rate of about 1 mm per day.
For example, a CRL of 23 mm would correspond to a gestational age of 9 weeks and two days (6 weeks plus 23 days = 9 weeks and 2 days).
The fluid collection is however increased in many fetuses with Down's syndrome and many other chromosomal abnormalities.
It is called a 'translucency' because on ultrasound this appears as a black space beneath the fetal skin.
It is this black space that you will see measured during the ultrasound scan.
Currently the most accurate non invasive test for detecting Down syndrome during pregnancy is the measurement of the nuchal translucency with an ultrasound between 11 to 14 weeks of pregnancy.