Objective: To establish the centile chart for the ratio of transverse cerebeller diameter to abdominal circumference throughout pregnancy from 13-40 weeks
Design: Cross-sectional study.
Setting: Department of Maternal - Fetal Medicine, Faculty of Medicine Siriraj Hospital, Mahidol University.
Subjects: The participants are women with gestational age between 13 and 40 weeks who attend Antenatal Clinic, Siriraj Hospital.
Material and Method: A total of 643 pregnant women were recruited in this study. The exclusion criteria were uncertained date of last menstrual period, multiple pregnancies, fetal abnormalities and maternal conditions which could affect fetal growth. Transverse cerebellar diameter and abdominal circumference of all the subjects were measured out by gestational age. The approximately equal numbers of fetuses were measured at each week of gestation. The mean and standard deviation of TCD/AC ratio is estimated at each
week of gestation. In addition the 5th, 10th, 50th, 90th and 95th percentiles were calculated at each week of gestation.
Results: Of 643 pregnant women, 149 (23.1%) were scanned before 20 weeks of gestation and 286 (44.5%) were scanned between 20 and 30 weeks of gestation. The last group was intervened after 30 weeks of gestation. TCD/AC ratio slowly declined from early pregnancy until about 20 weeks then remained stable. After 30 weeks of gestation it started to decline again.
Conclusion: The normal reference value of TCD/AC ratio in Thai fetus were shown in this study. This ratio can be used to calculate gestational age in intrauterine growth restriction fetus.

Keywords: TCD/AC ratio, Reference centile chart , Thai fetus