The definitive diagnosis of chromosomal abnormalities of the fetus is clearly understood by amniocentesis or chorionic villus sampling (CVS). However, the probability of the fetus falling in these methods is around 1 / 100-100%. Because of this possibility, expectant mothers may not be interested in such invasive methods.
During the NT measurement, a blood test to the expectant mother shows abnormal fetal protein or hormone levels. And in this case the fetus is at high risk. Therefore, it may be necessary to undergo such invasive tests.
In the diagnosis of Down syndrome, maternal age and NT measurement between 11-14 weeks started to be applied in 1990 and 75% of affected fetuses can be detected with this method. The probability of spontaneous loss of fetuses diagnosed as positive by the 12th week is around 30%. NT measurement also serves to diagnose Turner syndrome, trisomy 18, 13 and triploidy.
The measurement is ideally done between 11-14 weeks. The fetus image should cover 75% of the ultrasound screen. The fetus head should be in the neutral position. That is, it should not be bent forward or backward. Measurement is performed on the fetus lying on its back. The measurement should be made considering the thickest distance. It is very important that the amniotic membrane at the bottom does not participate in the measurement.
The baby's head-butt distance is also important. NT thickness is determined to be increased in 60-80% of trisomic fetuses. At 12 weeks, the average thickness was 2,18 mm. In 13% of chromosomally normal fetuses, this measurement may be over 1.5 mm. It is particularly important to combine NT values with blood values.
Especially in the fetus affected by Down syndrome, increased HCG and decreased PAPP-A are detected in the blood sample taken from the mother. Evaluation with blood values reduces the false positive rate.
In Down syndrome, only NT measurement determines the problem in 62% and it is false positive in 5%. When combined with blood values, these values are 73% and 4.7%, respectively. Detection rate was 89.7% in the second 3 months with a detailed ultrasound scan, while false positivity was around 4.2%.
The absence of the nasal bone of the fetus in the NT measurement determines the diagnosis of Down syndrome with a 95% probability.
In the light of all this information, CVS or amniocentesis should be performed for confirmation. NT measurement is performed only at the specified weeks since fetal lymphatic system develops and reflects the pathology immediately. Congenital lymphedema is mentioned if the nuchal thickness increases progressively between 11-14 weeks.