BackgroundSeveral studies show that gestational estimation during third trimester using fetal transcerbellar diameter (TCD) is superior to that measured by fetal biometry (BPD, HC, AC, and FL). However, the conclusions of studies stem from findings of correlation and regression statistical tests, which is not the recommended statistical analysis method for comparing the values of one variable as measured by two different methods. ObjectiveTo compare the accuracy of gestational age estimation by TCD to that by fetal biometry during the third trimester using Bland-Altman statistical analysis. MethodsThis was a cross-sectional study on pregnant women who presented for routine antenatal care follow-up in the third trimester (28-41 weeks) at St. Paul's Hospital Millennium Medical College (Ethiopia) between November 1, 2020, and February 28, 2021. Data were collected prospectively using a structured questionnaire on Open Data Kit (ODK). The primary outcome of our study was mean bias of gestational estimation (error in estimating gestational age) using TCD and fetal biometry (composite GA). Data were analyzed using STATA 15.p.l.c. Simple descriptive analysis, Bland-Altman(B&A) analysis, and Kendall's τa discordance measurement were performed as appropriate. Mean bias(error) and limits of agreement were used to present the finding significance. ResultsA total of 104 pregnant women in the third trimester were included in the study. The mean error(bias) when TCD was used to estimate the gestational age was 0.65 weeks versus a bias of 1.1 weeks by composite biometry, compared to gold standard method from crown-lump length (CRL) (in both cases). The calculated estimated limits of agreement was narrower in the case of TCD than by composite-fetal biometry (-3.56 to 2.25 vs -4.73 to 2.53). Kendall's τa discordance measurement revealed that gestational age estimation using composite biometry and that calculated from CRL were 51% to 70% more likely to agree than disagree whereas gestational age calculated from TCD and CRL were 62% to 77% more likely to agree than to disagree (P-value= <0.001). ConclusionGestational age calculation using TCD is more accurate than using composite gestational age (BPD, HC, FL, and AC) and it should be utilized to date third trimester pregnancies with unknown gestational age (unknown LNMP with no early ultrasound milestone).