This is a routine ultrasound examination done at 10 to 14 weeks of gestation.During the examination, the fetus is seen by abdominal ultrasound.The view of the nasal bone should show three distinct lines: The first two lines, are horizontal and parallel to each other, resembling an 'equal sign'.
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).
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).
This test results in a 9095% detection rate of pregnancies that may be at an increased risk of carrying a genetical problem such as Downs Syndrome.
If you are unsure of your period dates or have an irregular cycle, we suggest that you contact your doctor, midwife or health care provider as early as possible to arrange for a dating scan.
In the past few years it has been seen that approximately 3 in 4 babies with Down's syndrome do not have a visible nasal bone at the time of the first trimester screening test.
If the nasal bone is visible at the scan then this will reduce the chance of your baby having Down's syndrome.The CRL (see image above between the two white marked 1) is highly reproducible and is the single most accurate measure of gestational age.Charts have been developed for this purpose, but some simple rules of thumb can also be effectively used.Otherwise a scan is generally booked in the first trimester to confirm pregnancy, exclude ectopic or molar pregnancies, confirm cardiac pulsation and measure the crown to rump length for dating.The 12 week scan is a routine ultrasound examination carried out at 10 to 14 weeks of gestation.From 6 to 11 weeks gestational age, the fetal CRL grows at a rate of about 1 mm per day.