The benefits of prenatal diagnosis of congenital anomalies particularly in fetal conceived by  assisted reproductive technology   (ART) methods are widely known. The need for more accurate and noninvasive prenatal screening such as NT measurement is extra important in the case of assisted conception due to more concern over associated risks of invasive test such as amniocentesis and chorion villus sampling(CVS).Furthermore, the some noninvasive test such as triple test is not the  effective screening test in multiple pregnancies that is common between ART mothers .In the mid 1960s, amniocentesis was offered as pregnancy screening for fetal aneuploidy. Maternal age was used as the criterion for screening .New approaches in maternal serum and ultrasound screening has made it possible to assess the risk of  having a fetus with chromosomal and non chromosomal anomalies evenif conventional karyotyping is normal. Several studies have been published ,since the first report on the ability of nuchal translucency measurement to detect pregnancies affected by Down's syndrome by Nicolaides et al in 1941 (6). In the last 10 years, several studies have been declared that the positive predictive value of the NT measurement is high enough in both high-risk and low-risk groups, that patients with increased NT should be offered a detailed fetal evaluation (genetic sonography) at the end of the first trimester and also at 18-22 weeks of gestation. Prenatal diagnostic testing such as triple test, amniocentesis, fetal echocardiography and follow-up after birth is necessary. Measurement of the NT thickness combined with biochemical markers has a false-positive rate of 5%(7) and it is regarded as a basic screening test with high sensitivity for identifying fetuses at risk for aneuploidy(2,8,9).However ,NT is increased in 4.4% of euploid fetuses who are at risk for fetal anomalies and an adverse pregnancy outcome(9).

The risk of perinatal outcomeincreases in an exponential fashion as the NT measurement increases(10).The chance of delivering a healthy baby decreases with NT thickness from approximately 70% for an NT of 3.5–4.4 mm to about 15% for an NT of 6.5 mm or more (10).

Using NT screening to identify fetuses at risk for congenital anomalies at 11 to 14 weeks' gestation offer an option of early termination which is safer, and less traumatic in the expectant mother.

In this study there is only one case of anaploidy ( trisomy 18) with increased NT and in contrast to previous studies  no significant relationship could be derived between NT and chromosomal anomalies (P-value>0.99) probably due to small sample size.

According to the results of this study a significant relationship was found between a high NT and occurrence of malformations.