Screening for fetal trisomies by maternal age and fetal nuchal translucency thickness at 10 to 14 weeks to gestation

Ben W. Mol, Eva Pajkrt, Jan J.M. van Lith, Catia M. Bilardo

Research output: Contribution to journalArticlepeer-review

5 Citations (Scopus)


We read with interest the study of Pandya et al. (Vol 102, December 1995)1. The authors use the maternal age related risk for trisomy 21 at twelve weeks of gestation to calculate detection rates and false positive rates in the study group and the whole population in England and Wales. Nuchal translucency thickness measurement (NT) appears to be a useful test in the screening for trisomy 21. The authors continue by describing the combination of maternal age and NT as risk indicators in screening for trisomy 21. They calculate the post test odds for the presence of a trisomy 21. Likelihood ratios for each NT are calculated by comparing them with a reference standard. The reference standard in this study is either fetal karyotyping in early pregnancy or pregnancy outcome. As we understand from the methodology described, karyotyping is performed because of an ‘increased’ NT (n= 880), advanced maternal age (n= 1401), a family history of chromosomal abnormalities (n= 54), parental anxiety (n= 288) or an abnormal triple test result (n= 78). This is worrisome since the age-specific probability of the presence of trisomy 21 at time of karyotyping is doubled compared with the probability of the presence of trisomy 21 at term2. Women with a chromosomally abnormal fetus and an ‘increased’ NT are always karyotyped and therefore the abnormality is always detected. In contrast. women with a chromosomally abnormal fetus without an ‘increased’ NT are not always karyotyped. Approximately 50% of these pregnancies will result in the birth of a chromosomally abnormal child, whereas the other 50% will end in pregnancy losses. Since the spontaneously ended pregnancies are excluded from the analysis, this study might overestimate the sensitivity of NT measurement.

We emphasise the importance of the study. However, we want to stress that determination of the discriminative capacity of a test and evaluation of its subsequent implementation should not take place in the same population.
Original languageEnglish
Pages (from-to)1051-1052
Number of pages2
JournalBJOG: An International Journal of Obstetrics and Gynaecology
Issue number10
Publication statusPublished - Oct 1996


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