The nuchal translucency scan aims to measure the area of fluid behind the fetal neck. When combined with biochemical markers in maternal blood, this provides information to assess the chance of the baby having Down syndrome (trisomy 21), Edwards syndrome (trisomy 18) and Patau syndrome (trisomy 13).
The age of the woman is also used in the calculation.
The biochemical markers measured are:
Free beta human chorionic gonadotropin (bhCG) Pregnancy associated plasma protein-A (PAPP-A)
The eligibility criteria for the combined test is when the CRL is between 45 mm and 84 mm.
The length of the fetus or crown rump length (CRL) is also used in the combined calculation of the chance.
The combined test is only available for singleton and twin pregnancies.
For twin pregnancies, where there is second pregnancy sac containing a non-viable fetus the combined test cannot be done.