Ultrasonographic assessment of fetal epiphyseal ossification centres for estimation of gestation age in the third trimester.

Abstract

Background Ultrasonography plays an integral role in the assessment of gestational age in obstetric patients. The anthropometric measurements that are routinely used to estimate gestational age in the third trimester are not precise due to wide variability in foetal size. Evaluation of epiphyseal ossification centres (EOCs) can be a promising technique for estimating gestational age in the last trimester. Objective To evaluate the age at which foetal epiphyseal ossification centres appear and their size in various ages of gestation and to establish reference charts for the Kenyan population. Materials and methods A prospective cross-sectional study was done at Kenyatta National Hospital radiology department and antenatal clinic between November 2019 and March 2020. The size of the foetal distal femoral, proximal tibial and proximal humeral epiphysis ossification centres was measured during a routine obstetric ultrasound examination done on 297 normal pregnant women between 28 to 40 menstrual weeks. Data was analysed using SPSS version 23 and Microsoft Excel and was presented on tables and graphs. The mean age of appearance and size of the three epiphyseal ossification centres in each week of gestation was evaluated. Results The distal femoral epiphysis is not visualized before 30 weeks gestation but is observed in 72% of foetuses at 33 weeks, 86% at 35 weeks and 100% at 37 weeks. The proximal tibial epiphysis is seen for the first time at 31 weeks of gestation and in half of the foetuses at 35 weeks, 83% at 38 weeks, and 100% at 40 weeks of gestational age. The proximal humeral epiphysis is not observed before 36 weeks of gestation but is visualized in 30% of the foetuses at 38 weeks, 50% at 39 weeks and in 80% at 40 weeks. The mean diameter of the distal femoral epiphysis at 30, 35 and 37 weeks is 0.84mm, 3.40mm and 4.10mm respectively. The mean diameter of proximal tibial epiphysis increases with increment in gestational age, it measures 0.47mm at 31 weeks, 1.77mm at 35 weeks and 2.78mm at 37 weeks. At 36 weeks the proximal humeral epiphysis measures 0.33mm, it gradually increases in size to 0.84mm and 2.38mm respectively at 38 and 40 weeks. The percentage of ossification centres detected increases with increment in gestational age determined through ultrasound biometric measurements. Conclusion The mean age of appearance of distal femoral epiphysis, proximal tibial epiphysis and the proximal humeral epiphysis are 32, 35 and 37 weeks respectively. The distal femoral epiphysis, proximal tibial epiphysis and the proximal humeral epiphysis have a high positive predictive value for estimating gestation age of 35, 38 and 40 weeks respectively. Reference charts of gestational age using the size of distal femoral epiphysis, proximal tibial epiphysis and the proximal humeral epiphysis were drafted. Ultrasonographic detection of the distal femoral epiphysis, proximal tibial epiphysis and the proximal humeral epiphysis is a valuable marker of foetal age in the third trimester. RECOMMENDATIONS A case series study with follow up from the first trimester would increase accuracy of gestational age and provide a clearer understanding of appearance and growth of EOCs in the Kenyan population. A larger study should be carried out to confirm usefulness of EOCs as markers of gestational age and foetal development in normal pregnancy as well as in conditions that affect foetal growth and development.