Quantitative and qualitative study of fetal posterior fossa during the first trimester in a Chinese population

Background: To establish the normal reference ranges for parameters related to the fetal posterior fossa in the first trimester (11 ~ 13 + 6 weeks of gestation) and to analyze the relationship between them and crown-rump length (CRL) among the Chinese population. Methods Singleton pregnancies of 11 ~ 13 + 6 weeks (CRL:45–84 mm) with both parents from China were randomly selected from January 2021 to November 2021. The related parameters of the posterior fossa including cisterna magna (CM), intracranial translucency (IT), brain stem (BS), brain stem to the occipital bone (BSOB), and brain stem/brain stem to occipital bone (BS/BSOB) were measured in nuchal translucency (NT) median sagittal section clearly showing the fetal posterior fossa structure. 50 pregnant women of them were randomly selected to have the relevant values measured once again while another measurement was taken by a senior physician. Results This study included 1663 fetuses. The ICC (95% CI) of intra-operator reproducibility of CM, IT, BS, BSOB, BS/BSOB were 0.981 (0.952 ~ 0.991, P < 0.001), 0.929 (0.866 ~ 0.961, P < 0.001), 0.970 (0.946 ~ 0.983, P < 0.001), 0.991 (0.974 ~ 0.996, P < 0.001), 0.939 (0.892 ~ 0.965, P < 0.001), respectively; The ICC (95% CI) of inter-operator reproducibility 0.926 (0.860 ~ 0.960, P < 0.001), 0.810 (-0.083 ~ 0.940, P < 0.001), 0.820 (0.645 ~ 0.904, P < 0.001), 0.804 (0.656 ~ 0.888, P < 0.001), 0.772 (0.599 ~ 0.871, P < 0.001), respectively. There was a linear correlation between CRL and the parameters related to the posterior fossa (CM, IT, BS, BSOB, BS/BSOB). CM (mm) =-1.698 + 0.532 × CRL (cm) (r = 0.829, P < 0.001); IT (mm) = 0.701 + 0.179 × CRL (cm) (r = 0.548, P < 0.001); BS (mm) = 0.403 + 0.349 × CRL (cm) (r = 0.716, P < 0.001); BSOB (mm) = -0.277 + 0.719 × CRL (cm) (r = 0.829, P < 0.001); BS/BSOB = 0.747 − 0.021 × CRL (cm) (r = 0.196, P < 0.001). Conclusions It was feasible to observe fetal posterior fossa structures in the first trimester. We constructed normal reference ranges of CM, IT, BS, BSOB, and BS/BSOB. Furthermore, CM, IT, BS, and BSOB were all positively correlated with CRL, but BS/BSOB was negatively correlated with CRL.