Accuracy of Transcerebellar Diameter in Estimation of Gestatinal Age in Third Trimester and its Comparison with Conventional Parameters Such as Biparietal Diameter and Femur Length
Dr. Krithika*1, Siri Vummaneni 2, Shabana Babu 3
*Correspondence to: Dr. Krithika MBBS, MS, DNB, OBGYN. Department of Obstetrics and Gynaecology. Tamil Nadu.
© 2024 Dr. Krithika. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Received: 20 August 2024
Published: 10 September 2024
Summary
Obstetric sonography was performed in 100 pregnant women with uncomplicated pregnancy to evaluate the efficacy of TCD as a measure to calculate the predicted gestational age. Gestational age ranges from 28 weeks to 40 weeks. Only patient with known LMP, previous history of normal menstrual cycle and without any exclusion criteria were included in the study. Foetal biometry evaluated includes BPD, FL and TCD.
TCD was correlated with other foetal biometric parameters and clinical gestational age and the correlation was found to be significant. Nomogram of the TCD shows that there is a linear relationship between the cerebellar growth and the gestational age. So TCD can be used as a reliable parameter for determination of gestational age.
Abbreviations
CGA - Clinical Gestational Age
TCD - Transcerebellar Diameter
BPD - Biparietal Diameter
FL - Femoral Length
AC - Abdominal Circumference
HC - Head Circumference
IUGR - Intrauterine Growth Restriction
AFI - Amniotic Luid Index
CSF - Cerebrospinal Fluid
LMP - Last Menstrual Period
Introduction
Obstetric sonography plays an important role in the accurate determination of intrauterine gestational age. Knowledge of gestational age is important in following ways [1]: i) To anticipate normal spontaneous delivery or to plan elective delivery within the time frame of a term pregnancy. ii) To consider invasive procedures such as chorionic villus sampling, genetic amniocentesis and in interpretation of biochemical tests such as maternal serum alpha-fetoprotein screening. iii) To evaluate the foetal growth. iv) Gestational age influence the management decision if the foetus is diagnosed with anomaly. So all important clinical decisions are influenced by the gestational age.
Before the advent of sonography gestational age was calculated with the help of i) menstrual history ii) physical examination of the uterine size. iii)estimation of gestation age ossification center development by Xray. But these parameters have high variability.[1]
The last two decades have seen a tremendous progress in application of ultrasound as a diagnostic modality revolutionizing the management towards better care. This is particularly due to its non-invasive, non-ionizing nature and easy availability besides its cost effectiveness leading to wider acceptability. The exemplary safety record of diagnostic ultrasound is probably an important reason that it has become so widely used.[2] Ultrasound is safe for the patient, the foetus and the sonologist. There is no reported risk of ionizing radiation as in radiography,[3] or any other known biological or embroytoxic effect. It does not require any injections as sometimes needed in imaging studies.[4] The single or repeated intrauterine exposure to ultrasound, early or late in pregnancy does not carry the known risk of development of lymphatic or myeloid childhood leukemia,[5] as with X-rays. It is not associated with any harm to early fetal life, growth and vision or hearing during childhood.[6] Similarly no adverse effects have been observed on neurological development and subsequent school performance of the children.[7]
Sonographic measurement of foetal biparietal diameter is a well accepted predictor of gestational age. [8,9] However there is a high variability in the calculated gestational age which increases as pregnancy progresses with maximum difference approximating 3.6 weeks in the third trimester.[10,11] The estimation of gestational age from individual parameters like the HC, AC, and FL also shows a similar variability. Using all the above parameters this variability can be reduced by 25% to 30 %.[12] There are conditions like oligohydromnios, multiple gestation, breech presentation and intrauterine growth restriction (IUGR) that can alter the shape of the foetal skull which in turn can affect the BPD and increase the variability.[13]
Multiple gestations and IUGR can also affect the abdominal and femoral measurement. The present study is being undertaken to measure the transverse cerebellar diameter (TCD) to validate it as additional morphological measurement of foetal growth with less variability. The cerebellum and posterior fossa are aligned perpendicular to the plane of maximum extrinsic compression. Hence they are able to withstand deformation by extrinsic pressure than the parietal bones 13 and can be a more accurate parameter for the determination of gestational age.
Aim & Objectives
Objective of the Study
To perform obstetric ultrasonography in healthy women with uncomplicated pregnancy between the 28th week of gestation and 40 weeks to determine the correlation between the transverse cerebellar diameter and the gestational age as determined by the last menstrual period and other Sonographic parameters like biparietal diameter and femur length.
Materials and Methods
A cross sectional study was done in 100 healthy women with uncomplicated pregnancy between the 28th week of gestation and 40th week of gestation in outpatient and in-patient sections of Obstetrics and Gynaecology department of SRM Hospital and research Institute, Kattankulathur, Chennai.
This study was undertaken to determine a correlation between the transverse cerebellar diameter and the gestational age as determined by the last menstrual period and other sonographic parameters like biparietal diameter and femur length.
Inclusion Criteria
Healthy women with uncomplicated pregnancy between the 28th week of gestation and 40th week of gestation.
Exclusion Criteria
Ethical Clearance
The study required performing obstetric ultrasonography in normal pregnant women. Ethical clearance was obtained from the Ethical Review Committee of SRM Medical College.
Examination Method
All the statutory requirements under PNDT act were followed. All relevant clinical history was obtained and the correct LMP was confirmed. Transabdominal ultrasonography was performed with patient in supine position. Good acoustic coupling was obtained using synthetic ultrasound gel.
Obstetric ultrasonography were performed using a 3.5 MHz convex probe. Images were recorded in the thermal films.
In all the patients following parameters were obtained - BPD, HC, AC, FL, TCD, foetal heart rate, estimated foetal weight, AFI and placental position.
Plane used for measuring biparietal diameter and head circumference were through the third ventricle and thalami. Cavum septipellucidi must be visible in the anterior portion of the brain and the tentorial hiatus must be visible in the posterior portion of the brain. The cursors are positioned in outer edge of near calvarial wall to inner edge of far calvarial wall for BPD.
The femoral length was obtained by aligning the transducer to the long axis of the diaphysis. Measurement cursors are placed at the junction of the cartilaginous epiphysis and bone and the thin bright reflection of the cartilaginous epiphysis should not be included.
Transverse cerebellar diameter is obtained in the axial plane in the cerebellar view i.e with a slight rotation of the transducer approximately 300 from the conventional thalamic plane where the biparietal diameter is measured using the cavum septipellucidi, third ventricle and thalami as landmarks. In this plane posterior fossa with cerebellum is visualized. The cisterna magna is just posterior to the cerebellum. This plane provides the widest transcerebellar diameter.
Gestational age for the measured TCD is obtained from the reference chart “Predicted menstrual age for transverse cerebellar diameter of 14 mm to 56 mm”by Hill et.al., in his study “Transverse cerebellar diameter as a predictor of menstrual age” in 1990.[14]
Method of Statistical Analysis
The following methods of statistical analysis have been used in this study.
The results for each parameter (numbers and percentages) for discrete data and averaged (mean + standard deviation) for continuous data are presented in Table and Figure.
An evaluation was made of the linear relationship between two variables using Pearson's correlation. Pearson Correlation technique used to test the direction and strength of the relationship between gestational age obtained using transverse cerebellar diameter vs individual parameters such as biparietal diameter and femoral length. It uses the statistic R which falls between -1 and +1. The + and – signs are used for positive linear correlations and negative linear correlations, respectively. A correlation greater than 0.8 is generally described as strong, whereas a correlation less than 0.5 are generally described as weak.
Results
The age of 100 patients included in the study was in the range of 20 to 35 years with the mean age of 27.37 years. Out of 100 patients included in the study, 30 (30%) werein the age group of 20-25 years, 51 (51%) were in the age group of 25-30 years and 19(19%) were in the age group of 30-35 years.
Discussion
Accurate gestational dating is of paramount importance and the cornerstone for management of pregnancies. Methods to date pregnancies should be simple and straightforward, in all gestational ages. Accurate and easily reproducible sonographic foetal biometric parameters for gestational dating are clinically important for the optimal obstetric management of pregnancies. This is especially true in determining timing of a variety of gestational tests, assessing adequacy of growth and timing of delivery for the optimal obstetric outcome.
In this prospective study of 300 healthy women with uncomplicated pregnancy a correlation is suggested between the gestational age and TCD. A linear relationship was found during the second (12 wks to 24 wks) and third (24 wks to term) trimester between the cerebellar growth measured in mm (millimeters) and the gestational age in weeks. This relationship of foetal cerebellar growth and gestational age is statistically significant.
Many studies have been conducted to assess the variability in gestational age determination from TCD in second and third trimester. In the reported studies21,61,63,64,74 this linear relationship has been established in second and third trimesters correlating well with clinical gestational age. In the present study TCD correlates well with gestational age with high correlation coefficient of 0.96 in the third trimester along with FL, as the first accurate parameters for assessing the gestational age. In second trimester, even though the correlation coefficient is significant with 0.89 the correlation coefficient is slightly less than with the other parameters. However TCD is also one of the significant measurement to be considered.
From a biological perspective cerebellum is not liable to change in form and size because of dense surrounding petrous ridges and occipital bone. This is at variance with several other biometric parameters, especially abdominal circumference, which may be drastically altered by extremes of fetal growth. Hence TCD can be eminently used where it is not possible or difficult to measure BPD or in cases where there are variations in size and shape of foetal head.
TCD has also been measured to predict mean gestational age in different ethnic groups. Foetal TCD is not independent of ethnic origin of patient. Nomogram for TCD can be developed for different countries and races to predict gestational age for a particular ethnic population. In our study, all the patients were of Indian origin and the normogram for predicting gestational age from TCD was obtained. The values were compared with a study conducted by Hill et. al.[14] and it was observed that the values reported in our study are slightly smaller. This is probably due to the difference in the ethnic origin of patients.
In the study by Neryet. al., the correlation of TCD with BPD, HC, AC and FL were statistically significant with the P value of 0.92, 0.92, 0.89 and
0.90 respectively.74 Similarly in the present study the correlation of TCD with other foetal biometric parameters such as BPD and FL were statistically significant with the P less than 1.
The results of present study and previously published studies on TCD show that additional small improvements in accurate gestational dating can be achieved by incorporating the results of TCD with some combination of other fetal biometric parameters, including biparietal diameter, head circumference, abdominal circumference, and femur length. Nevertheless, the best combination of biometric measurements remains to be determined. We recommend that TCD be used as an important sonographic biometric parameter for accurate prediction of GA.
Conclusion
References