NT should be performed between 11 weeks+ 4 days and 13 weeks + 5 days. (This is when the embryo becomes fetus). In this period we may assess not only the risk of chromosomal defects but also we may study the fetal morphology anticipating to a certain extent the II trimester scan.




Over 100,000 pregnancies have been assessed at the Fetal Medicine Foundation, London, between 11 and 14 weeks. Similar sudies have been conducted in all the major pre-natal diagnosis centres all over the world with similar results. Overall more than 350,000 pregnancies have been assessed.

The advantage of first trimester screening is to identify those patients at higher risk of having a baby with a problem (chromosomal) and offer them diagnostic testing (CVS or amniocentesis). Early fetal assessment also allows to detect other (non chromosomal) anomalies because assessment of fetal anatomy is performed at an earlier gestation.