Nuchal translucency

The optimal gestational age for measurement of fetal NT is 11⁺⁰ to 13⁺⁶ weeks. The minimum fetal crown–rump length (CRL) should be 45 mm and the maximum 84 mm.

The reasons for selecting 11⁺⁰ weeks as the earliest gestation are:

• Screening necessitates the availability of a diagnostic test and chorionic villous sampling before this gestation is associated with transverse limb reduction defects.

• Many major fetal abnormalities can be diagnosed at the NT scan, provided the minimum gestation is 11 weeks.

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

• To provide women with affected fetuses the option of 1st rather than 2nd trimester termination.

• The incidence of abnormal accumulation of nuchal fluid in chromosomally abnormal fetuses decreases after 13 weeks.

• The success rate for taking a measurement decreases after 13 weeks because the fetus becomes vertical making it more difficult to obtain the appropriate image.