Start Dating rules 1 week

Dating rules 1 week

The crown rump length is a universally recognized term, very useful for measuring early pregnancies.

This is normally less than 2.5mm and when seen increased (greater than 2.5mm, see image on the right) may indicate the baby has Down syndrome or may indicate another chromosomal abnormality. The nuchal translucency test will also check whether your baby has a visible nasal bone.

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.

Occasionally the nasal bone cannot be seen at 11 weeks as it is too early in the pregnancy.

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.

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.

Crown Rump Length: Gestational age = 6 weeks plus (CRL x days).