Abstract： Objective To explore the application value of prenatal three-dimensional ultrasound measurement of thymic parameters in the evaluation of fetal growth restriction (FGR).Method Seventy-five pregnant women with FGR in the third trimester of pregnancy who were admitted to the Northwest Women's and Children's Hospital from March 2021 to March 2022 were selected as the study group.Seventy-five pregnant women with normal fetal development in the third trimester of pregnancy who were admitted to the same hospital during the same period and in the same age group were selected as the control group using a random number table method.All participants underwent prenatal three-dimensional ultrasound examination to measure the thymuic volume,transverse diameter,anterior and posterior diameter,and thymic-thoracic ratio (T-T) value of the fetus.Clinical data,FGR-related serological markers[(placental growth factor (PLGF),pregnancy-associated plasma protein-A (PAPP-A),β-human chorionic gonadotropin (β-hCG)],fetal abdominal circumference Z-score,uterine artery flow parameters[pulsatility index (PI),resistance index (RI),end-systolic peak flow velocity to end-diastolic peak flow velocity ratio (S/D),fetal three-dimensional ultrasound thymic parameters were compared between the two groups,and the relationship between fetal three-dimensional ultrasound thymic parameters and other FGR related indicators were analyzed.The relationship between fetal thymus parameters and 5-min Apgar score of fetus was analyzed and the value of fetal FGR was evaluated.Statistical methods performed by t-test,x2 test,Spearman correlation analysis and receiver operating characteristic (ROC) curve.Result There was no significant difference in age,gestational age and postpartum between the study group and the control group (all P>0.05).Prenatal body mass index (BMI)[(21.4±1.9) vs (23.2±2.2) kg/m2],serum PLGF[(0.63±0.19) vs (1.04±0.30) MOM],PAPP-A[(0.78±0.23) vs (1.57±0.42) MOM,the levels of β-hCG[(0.69±0.21) vs (1.35±0.36) MOM]and Z-score of fetal abdominal circumference[(-1.75±0.53) vs (-0.16±0.05) points]in the study group were lower than those in the control group.Incidence of gestational diabetes mellitus (GDM)[26.7% (20/75) vs 6.7% (5/75)],the incidence of hypertensive disorders of pregnancy (HDP)[(34.7% (26/75) vs 10.7% (8/75)],uterine artery PI (1.39±0.37 vs 0.95±0.30),RI (0.71±0.21 vs 0.54±0.16),S/D (3.08±0.85 vs 2.49±0.71) in the study group were all higher than those in the control group.The differences were statistically significant (all P<0.05).In the study group,the thymic volume[(8.25±0.76) vs (8.84±0.83) ml],transverse diameter[(3.11±0.25) vs (3.40±0.28) cm],anterior and posterior diameter[(1.33±0.15) vs (1.45±0.20) cm,T-T values (0.32±0.06 vs 0.38±0.08) were lower than those in the control group (all P<0.001).Spearman correlation analysis showed that the thymic volume,transverse diameter,anterior and posterior diameter,and T-T value of the fetus in the study group were positively correlated with serum PLGF,PAPP-A,β-HCG levels,and Z-score of fetal abdominal circumference,and negatively correlated with uterine artery PI,RI,and S/D (P<0.05).The 5 min Apgar score of the study group was<7 and 7-10,the fetal thymic volume[(7.92±0.59) vs (8.45±0.65) ml],the transverse diameter[(2.95±0.22) vs (3.21±0.26) cm],the anterior and posterior diameter[(1.24±0.13) vs (1.38±0.15) cm]and T-T value (0.29±0.03 vs 0.34±0.05) in<7 groups were lower than those in 7-10 groups (all P<0.001).The fetal thymic volume,transverse diameter,anterior and posterior diameter and T-T value were positively correlated with 5 min Apgar score (r=0.348,0.416,0.379,0.450,all P<0.05).The maximum area under the curve (AUC) of fetal FGR was 0.918 (95% CI:0.863-0.957,P<0.001) in the combined evaluation of fetal thymic volume,transverse diameter,anterior and posterior diameter and T-T value.Conclusion The thymic parameters of prenatal three-dimensional ultrasound are closely related to the uterine artery blood flow,fetal placental development,and neonatal 5 min Apgar score in FGR.The combined assessment of fetal thymic volume,transverse diameter,anterior and posterior diameter,and T-T value has a high diagnostic efficiency for FGR.