The ratio of Prenasal thickness to nasal bone length in fetal screening ultrasonography at second trimester of pregnancy

chromosomal abnormalities is the cause of more than 50-80% of abortions and the reasons of 4-28% of all mental retardations. Determining the fetal karyotype by chorionic villus sampling and amniocentesis are the gold standard methods for diagnosis of these diseases. Since these procedures are invasive and include at least 1% risk of fetal abortion, in recent years, different screening tests based on sonographic and biochemistry indexes are introduced and developed to determine who needs these invasive diagnostic methods. This study was performed with aim to assess the ratio of prenasal thickness to nasal bone length (PT/NBL) in fetal screening ultrasonography at second trimester of pregnancy.
This cross-sectional, analytic and descriptive study was performed on 500 healthy pregnant women with singleton fetus at 15-26 weeks of gestation in 2015. Ultrasound measurements of PT and NBL were performed on a midsagittal plane of fetal face. These measures were recorded in statistical table. Data was analyzed with SPSS software (version 22) and Statistica (version 10). P The mean of PT increased from 2.23±0.47 mm at 15 weeks to 4.32±0.9 mm at 26 weeks of gestation (P=0.001). The median of NBL increased from 4±0.17 mm at 15 weeks to 8.35±0.56 mm at 26 weeks (P=0.001). There was a linear relationship between PT and NBL with gestational age. The mean of PT/NBL was 0.5±0.78 which was constant during these weeks (P=0.89).
Gestational age, maternal age and fetal sex had no influence on PT/NBL ratio (P>0.05). So, this ratio can be used as an independent predictive factor for aneuploidies.