The crown rump length is a universally recognized term, very useful for measuring early pregnancies.The CRL (see image above between the two white marked 1) is highly reproducible and is the single most accurate measure of gestational age.
The 12 week scan is a routine ultrasound examination carried out at 10 to 14 weeks of gestation.At the first trimester scan they confirm that the fetus is alive, they assess the gestational age by measuring the crown-rump length and will also look for any major problems.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.
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.
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.
The test cannot be performed outside this time frame as certain features that need to be seen on the scan are not present.