First trimester fetal biometric assessment is very important for the accurate estimation of gestational age to conduct optimal antenatal care. The timings any antenatal diagnostic tests, elective C-section, induction of delivery for postdated pregnancies are based on gestational age. This is the first study to construct the first trimester Mongolian fetal standard biometry which is to answer the following questions; which widespread first trimester fetal nomogram is similar to Mongolian fetal biometry, are there any unique fetal characteristics in Mongolian ethnic groups. In accordance with inclusion and exclusion criteria 658 healthy women between 11-14+6 weeks of gestation examined on the following fetal biometric measurements: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL). The study occurred between Jan 2013 to April 2014 Combined measurement of abdominal and vaginal examination was done. Physicians who worked for more than 10 years in the field of obstetrical ultrasound are required to do at least three measurements to get correct results.Statistical analysis was performed with SPSS-21, E-View, and Windows Excel. A regression model was developed with biometric measurements and was compared to widespread nomograms in the world.The obtained Mongolian first trimester standard fetal biometry was compared to Hadlock from Europe, M.A.Esetov from the same geographic region, Osaka from Asia, ASUM from the reference of Australia. BPD was 4-6mm longer, HC was 20-13mm longer than Hadlock but it was the same as others. FL was a bit shorter than Hadlock and ASUM but it was the same as Osaka and M.A.Esetov at 12-14+6 weeks of gestation. Difference with the obtained nomograms may be due to ethnic differences of the Mongolian fetal development. Current study shows that there are needs to use separate growth chart to estimate accurate gestational age for Mongolian population.

GoalThis study was performed to construct Mongolian ethnic specific crown-rump length (CRL)nomogram and to compare its ability to predict gestational age with previously published widelyused nomograms in the world.Materials and MethodsA regression model was developed for estimation of gestational age using CRL measurements of1170 singleton fetuses in the Mongolian population. Measurements were obtained by placing thecalipers of the ultrasound machine from the crown to the rump. The appropriateness of previouslyestablished widespread CRL nomograms for predicting the gestational age was assessed in theMongolian population to determine comparability between nomograms.ResultsCRL corresponds to other nomograms up to 9 weeks of gestational age. There is a 4-5mm differenceof Robinson Hadlock; 7mm of Osaka at 10-12 weeks but the same with Campbell, 4 mm long ofCampbell at 13 weeks.Conclusions:CRL measurements are used as a reliable method for estimation of the gestational age as well asa baseline for comparing gestational ages later. CRL corresponds to other widely used nomogramup to 9 weeks gestational age. There is a 3mm differences of M.A.Esetov, 4-5mm Robinson, andHadlock; 7mm of Osaka at 10-12 weeks but the same with Campbell, 4 mm long of Campbell at13 weeks. Difference with the established nomograms may be due to ethnic differences of theMongolian fetal development. After 13 weeks, CRL measurement is unreliable due to flexion of thefetus.