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Nuchal translucency screening in monochorionic twin pregnancies

Prenatal screening for chromosomal abnormalities using nuchal translucency (NT) assessment provides a prospective mother with the opportunity to obtain an accurate estimate of her individual and actual risk for having a fetus affected by trisomy 21. By being aware of her adjusted risk, she can exercise autonomy in order to make the most appropriate decisions regarding further diagnostic testing1,2. It is clear that in the field of prenatal screening, to be assigned a numerical risk rather than a positive or negative result is more meaningful for a pregnant woman and her doctor3. The policy to offer more widely such screening has had the further benefit of decreasing the number of prenatal invasive procedures offered on the basis of maternal age, following assignment of a low risk on ultrasound examination4. At the core of this ultrasound-based risk assessment is measurement of fetal NT, and the quantification of its thickness compared with the reference distribution of karyotypically normal fetuses. Rigorous trials, experiences gathered from various centers, careful training, assessment of competence and compulsory periodical audit of operators have allowed the widespread adoption of NT screening with effective results5,6. To date, however, the actual cause of the enlargement of the NT, that could be the key to distinguishing between karyotypically abnormal and normal fetuses, has not been clarified and remains a challenging enigma. There has been continuing uncertainty surrounding our knowledge of the causal mechanism of increased NT for the last 10 years and no breakthrough has come from the most recent literature7–9. Despite this, the NT test has become widespread. The NT test is effective in terms of accuracy for screening; it works well when considered alongside some other additional markers in a ‘contingent screening’ policy, and some clever refinements to the risk calculations have been proposed (e.g. when a nuchal cord is present a mean between the NT thickness above and below the cord is proposed10). Enlarged NT appears in fetuses affected by pathological conditions such as some chromosomal abnormalities, congenital heart defects, major extracardiac malformations and genetic syndromes11,12. Structural heart defects leading to temporary cardiac failure, delay in the development of the heart with reduced compliance, venous congestion of the head and neck, altered composition of the extracellular matrix and delayed or abnormal lymphatic system development are among the putative causes of increased NT. A compelling hypothesis is that a common denominator of these conditions leading to the enlargement of NT may be the phenomenon of heterochronia, possibly affecting trisomy 21 fetuses by a generalized delay in developmental milestones. Moreover, enlarged NT happens unexpectedly in some completely normal fetuses, and in cases of monochorionic diamniotic (and rarely monoamniotic) twins. In singleton pregnancies, enlarged NT (greater than the 95th centile) is encountered in about 5% of cases and in dichorionic pregnancies the frequency is about the same. However, in fetuses of monochorionic diamniotic pregnancies the frequency of an enlarged NT is significantly higher, about 15%, without epidemiological demonstration of a higher frequency of chromosomal abnormalities13,14.