The nomogram of prenasal thickness and nasal bone ratio and its relationship with nuchal fold in second-trimester fetal ultrasound.

Aim: We aimed to define the normal values of second-trimester fetal prenasal thickness and nasal bone length ratio (PNT/NBL) in a low-risk Turkish population and investigate the relationship between the increased PNT and nuchal fold (NF).

Method: We retrospectively reviewed the data of 650 cases considered to be euploid fetuses who presented to our clinic between December 2013 and September 2014 for a second-line ultrasound between 17 and 24 weeks. Their PNT, NBL, NF and biometric (BPD, HC, AC, FL and HL) measurements were recorded.

Results: A total of 650 cases considered to be euploid fetuses were included in this study. There was a linear increase in PNT with the gestational weeks (mean value 3.01 at weeks 17-18 and 3.76 at weeks 23-24). The PNT/NB ratio showed a slight decrease in advancing gestational weeks (mean value 0.57 at weeks 17-18 and 0.50 at weeks 23-24). The 95th percentile value of the PNT/NB ratio, independent of the gestational week, was 0.76. The values of both PNT and NF increased with advancing gestational weeks, but there was no statistically linear increase between PNT and NF (R : 0.115).

Conclusion: In this study, we provided a nomogram of the PNT/NB ratio. Both NF thickness and PNT should be dealt with as two separate markers. Although the formation mechanisms are thought to be the same in both of them, a strong linear relationship does not exist between them.