Fetal prenasal thickness and its correlated ratios between 16 and 23 weeks of gestation

Objective: The aim is to investigate the correlations and distributions of ratios of fetal prenasal thickness (PT), biparietal diameter/prenasal thickness (BPD/PT) and prenasal thickness/nasal bone length (PT/NBL) to the weeks of gestation (WG). Methods: Women with healthy singleton pregnancies between 16 and 23 weeks of gestation were included in the study. With the biometric evaluations of fetuses, the fetal structures were examined in details. Prenasal thickness and nasal bone were measured and BPD/PT and PT/NBL ratios were calculated in each case. The correlation between PT and the week of gestation was evaluated by regression analysis. PT percentiles according to the weeks of gestation were calculated. The correlations between BPD/PT and PT/NBL ratios and the weeks of gestation were investigated and their percentile values were calculated. Results: We included a total of 393 pregnant women in our study. It was found that PT increased significantly together with the week of gestation, and regression equation was calculated as PT = WG × 0.24 – 0.87; R=0.65; p<0.01. Mean PT was 3.9±0.7 mm and 95th percentile values were 4 mm at 16–18 weeks of gestation, 4.7 mm at 19 weeks of gestation, 5 mm at 20–21 weeks of gestation, and 6.0 mm at 22–23 weeks of gestation. When the correlation between PT and BPD was investigated, it was seen that there was a linear increase (R=64, p<0.01). However, there was no significant correlation between BPD/PT ratios and the week of gestation (p=0.29). Mean BPD/PT ratio was 12.3±1.6 and 5th percentile value was 10. A significant correlation was found between PT and NBL (R=0.63; p<0.01). According to the regression analysis, there was a negative correlation between PT/NBL ratio and WG (PT/NBL = -WG × 0.016 + 1.02; R=0.25; p<0.01). Mean PT/NBL ratio was 0.7±0.1 and 95th percentile values were calculated as 1.1–0.9 between 16 and 23 weeks of gestation. Conclusion: While PT and PT/NBL ratio changes together with the week of gestation between 16 and 23 weeks of gestation, BPD/PT ratio remains constant. Therefore, we think that using BPD/PT <10 (5th percentile) value in screenings could be more useful and practical.