Objective: To create a nomogram of intracranial translucency (IT) and to study the correlation between IT and crown-rump length (CRL), and between IT and gestational age (GA).

Materials and Methods: Thai singleton pregnant women between 11 to 13⁺⁶ weeks of gestation were enrolled. Fetal CRL, IT, biparietal diameter (BPD), brain stem diameter (BS), brain stem to occipital bone diameter (BSOB) and cisterna magna width (CM) were measured. Best fit equations between IT and CRL, and IT and GA were constructed. The correlation between CRL and remaining parameters were calculated.

Results: Two hundred eleven Thai pregnant women were enrolled. The mean GA was 12⁺³ weeks (SD 5 days). The IT ranged from 0.91 to 3.89 mm (SD 0.56). Curve estimation analysis demonstrated a quadratic relationship between IT and CRL [IT (mm) = 1.928701 – 0.027603*CRL + 0.000468*CRL² (R2=0.352, p<0.001)]. A quadratic relationship also provided the best fit between IT and GA [IT (mm) = 18.242997 – 0.425972*day + 0.002744*days² (R2=0.295, p<0.001)].

Conclusion: A nomogram of IT and other intracranial structures for open spina bifida screening at 11 to 13⁺⁶ weeks of gestation in Thai population has been established. The assessment of IT is feasible.

Keywords: Open spina bifida screening, Intracranial translucency, Brain stem diameter, Brain stem to occipital bone diameter, Cisterna magna width