The Ratio of Prenasal Thickness (PT) to Nuchal Fold Thickness (NFT) in Fetal Screening Ultrasonography at Second Trimester of Pregnancy and the Relationship with Maternal Age, Gestational Age and Fetal Sex

Regarding to the importance of the main role of ultrasonography in screening of chromosomal disorders, the present study was performed with aim to evaluate the relationship between NFT and PT in fetuses of second trimester of pregnancy; if these two ultrasonographic markers are correlated, PT can be used in fetuses in which NFT cannot be measured due to fetal position in the uterine.

In this descriptive-analytical and cross-sectional study, 500 pregnant women at second trimester of pregnancy underwent routine ultrasound scan of anomalies. First, pregnancy ultrasound along with determination of gestational age and general pregnancy information, as well as scan of anomalies including all fetal organs were done and fetal sex, maternal age and gestational age were recorded. Data analysis was done using SPSS (version 22) and Statistica (version 10) statistical software. P<0.05 was considered statistically significant.

The mean gestational age was 19 weeks and mean maternal age was 30.26 years. The mean of PT in pregnant women with gestational age of less than 19 weeks was 1.8±0.212 mm, and in pregnant women with gestational age of ≥19 weeks was 5.6±0.161 mm (p<0,001). The mean of NFT in pregnant women with gestational age of less than 19 weeks was 2.9 ±0.194 mm, and in pregnant women with gestational age of ≥19 weeks was 5.9±0.187 mm (p<0,001). The mean of PT according to maternal age was 4.4±0.32 mm, which increased from 2.7 mm under 20 years to 5.65 mm over 40 years (p<0.001). Also, the mean of NFT according to maternal age was 4.2±0.211 mm, which increased from 3.2 mm under 20 years to 5.3 mm over 40 years (p<0.001), so there was a significant relationship between PT and NFT with gestational age and maternal age; Thus, both increased with increasing gestational age and maternal age (p<0.001). PT/NFT ratio was stable according to gestational age (p=0.205), maternal age (p=0.111) and fetal gender (p=0.752). No significant relationship was observed between maternal age, gestational age and fetal gender with PT/NFT ratio (p>0.05).

It was found that NFT and PT as the ultrasonographic markers of aneuploidies are correlated with each other. It can be concluded that PT can be used in cases which NFT cannot be measured due to fetal position in the uterine.