Nuchal translucency

The fetal NT should be measured between 11 weeks and 13 weeks and 6 days. The minimum fetal Crown to Rump Length (CRL) should be 45 mm and the maximum 84 mm.

The reasons for selecting 11 weeks as the earliest gestation are:

• If after the screening test the parents choose to have CVS this is not safe before 11 weeks because it can cause limb defects
• Many major fetal abnormalities can be diagnosed at the NT scan, provided the minimum gestation is 11 weeks. With earlier scanning the abnormalities can be missed because the fetus is too small and the various organs are not developed enough to be visible

The reasons for selecting 13 weeks and 6 days as the upper limit are:

• To provide women with affected fetuses the option of 1st rather than 2nd trimester termination (the latter is more difficult and dangerous)
• Even in fetuses with chromosomal abnormalities the high NT usually disappears after 13 weeks