Crown Rump Length: Gestational age = 6 weeks plus (CRL x days).
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).
During the examination, the fetus is seen by abdominal ultrasound.
Occasionally the view is not clear and it may be necessary to perform a vaginal scan.
Accuracy of CRL after 12 weeks in predicting gestational age diminishes and is replaced by measurement of the width of the fetal head (biparietal diameter or BPD).
The nuchal translucency (also spelled nucal translucency) is a collection of fluid beneath the fetal skin in the region of the fetal neck and this is present and seen in all fetuses in early pregnancy.
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.
Most parents look forward to their scan because it gives them the first glimpse of their baby.The fluid collection is however increased in many fetuses with Down's syndrome and many other chromosomal abnormalities.