Introduction
The accurate understanding of gestational age (GA) is a keystone in an obstetrician's ability for antepartum care and management. Failure can result in iatrogenic prematurity which is connected with increased perinatal morbidity and mortality.[1] Ultrasonography of fetal measurements is highly reliable in the first and second trimester of pregnancy, but the reliability of any ultrasound method greatly diminishes as gestation advances. In the third trimester, the reliability of any single ultrasound parameter is poor.[123]
Since the last decade, ultrasound parameter transcerebellar diameter (TCD)' is considered a reliable predictor for GA in the third trimester.[45678] Size of the cerebellum is less affected by deviation in feta growth restriction or growth acceleration.[6789] The predicted GA by TCD between 22 and 28 weeks is within 0–2 days, between 29 and 36 weeks is within 5 days and at 37 weeks is 9 days of actual gestation. TCD nomogram predicts GA with the accuracy of 94% in the third trimester.[89]
TCD is one such nontraditional parameter for estimating GA under study. It is easy to identify and measure. It is strongly correlated to GA. This parameter is particularly useful in the prediction of GA in patients who are unsure of dates or suspected of having intrauterine growth restriction and it is a standard against which other parameters can be compared.
A true estimation of GA plays an important role in quality maternity care such as assessment of fetal growth and to schedule the delivery date. Any inaccurate estimation may lead to perinatal morbidity and mortality due to iatrogenic pre-or post-maturity. About 30% of women forget their accurate LMP or misunderstand early pregnancy bleeding as normal menses. GA can be estimated in the first trimester by ultrasonic measurement of diameter and volume of gestational sac as well as crown-rump length. Furthermore, other biometric indices such as fetal biparietal diameter (BPD), femur length (FL), abdominal circumference (AC), foot length, and head circumference (HC) are used for GA estimation at different pregnancy trimesters.
These parameters are limited in late pregnancy and in uncertain dated pregnancies. This was explained by the increasing biologic variability in advancing GA. To determine the abnormal fetal growth, serial ultrasonographic evaluation in the third trimester is required for appropriate fetal surveillance and intervention. The size of the cerebellum or TCD is important because it is a useful biometric parameter in estimating GA in the second and third trimester.
So we aim to establish reference curves for the size of the fetal TCD measured using ultrasound between the 25th and 32 weeks of gestation, determine the accuracy of fetal TCD measurement in the prediction of GA and to evaluate the application and accuracy of TCD in determining the GA of the fetus and Its Correlation.
Material and Methods
A cross-sectional prospective study conducted to determine the different ultrasonographic interpretations concerned. This study was a scan center based one in a hospital that conducted to determine the different parameters of the fetus in normal pregnancies. The study was conducted during the period between 2019 and February 2020. The study was conducted at Arogyamatha hospital in collaboration with the Department of ultrasound Scan center in Gengavally taluk, Salem district of Tamilnadu, India. The subset of antenatal mothers attending the Department of ultrasound scan of the Hospital for assessment of their feto-placental profile during their pregnancy. All the subjects were identified from the antenatal mothers attending the Department of ultrasound scan of the Hospital for assessment of their feto-placental profile along with documented first trimester crown-rump length (CRL) in previous ultrasonography report. Study subjects were selected according to the inclusion and exclusion criteria mentioned in the study protocol.
The data collection was carried throughout the weekdays at the scan center with the previous appointments. The subjects were examined once never repeated for participation in this study. Maintaining the criteria of inclusion and exclusion at all antenatal mothers was approached for this study based on the reviews of previous research papers.
Eligibility of the study population
Inclusion criteria
- Pregnancy having 25–32 weeks of gestation
- Having documented 1st-trimester investigations
- Uneventful pregnancy till date or without any bad obstetric history
- Given informed consent
- Pregnant women having single live normal fetus.
Exclusion criteria
- Having no documented 1st-trimester CRL
- High-risk pregnancy or in emergency crisis
- Documented or reported any associated disorders (medical/surgical) of the mother
- Documented with any fetal anomaly.
Study plan
After obtaining the approval from Ethical committee from the concerned institution, we had visited the center and started the study with the CRF forms. Data were collected from the participants at the time of ultrasonography and transferred to the electronic module. And we were advocated that every participant should visit the center between the 25th and 32 weeks (3rd trimester) of the gestational period.
The patient was asked to come with full bladder as it was difficult to measure the BPD as the head was sagging down in the pelvis. Ultrasound jelly was used to eliminate the air interface between the transducer and the patient skin. If jelly is not used, the presence of air causes hindrances during the examination. Sufficient jelly was spread over the abdomen before the examination. The TCD was mainly measured in the transcerebellar plane. The measurement of TCD was obtained by placing electronic calipers at outer to outer margins of the cerebellum. The landmarks of the thalami, cavum septum pellucidum and third ventricle were identified, thereby slight rotating transducer below the thalamic plane. The posterior fossa was revealed with the characteristic butterfly-like appearance of the cerebellum [as shown in the [Figure 1]. In all cases cerebellum was seen as two lobes on either side of the midline in the posterior cranial fossa.
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Figure 1: Ultrasonagram showing butterfly shaped cerebellum in the posterior cranial fossa
Each patient was subjected to serial ultrasound for fetal biometry and TCD measurement. The BPD, HC, AC, femoral length (FL), fetal heart rate (FHR) were collected along with TCD to assess the GA. For the data collection same machine, same probe and the same observer were maintained throughout the study period.
Statistical analysis
The data were entered on the space scheduled in the master sheet previously made ready. Variables were explored and analyzed using appropriate statistical methods. The data were entered in Excel datasheet and converted to SPSS (SPSS Incorporation Chicago, USA) data sheet and final analysis was done by using the SPSS (Statistical Package for Social Science) 12 versions. The statistical evaluation between fetal TCD and GA was assessed. Various parametric and nonparametric statistical analysis methods used like Descriptive analysis, Regression analysis, and Pearson correlation coefficient “r” in the range of (+1, −1) was calculated.[1011]
Results
Summary of the results is tabulated in [Table 1]. The total number of mothers who participated in this study was 100. The patients mean age was 24 years and mean GA depending on ultrasound was 28 ± 0.7 (27–30) weeks. The mean fetal biometry parameters including BPD of the study population was 73 ± 2.3 mm (67–80), HC was 264 ± 8.7 mm (237.8–311), AC was 244 ± 8.9 mm (226.6–265.9), FL was 55 ± 2.1 mm (49.5–59.6), FHR was 149 ± 8.5 beats (121–175). The mean transcerebellar measurement of the fetus was 31 ± 1.1 mm (range 28.6–33.9 mm) as shown in Figures 2 and 3. Regression and correlation coefficients for correlation between GA with routine fetal biometric parameters and TCD are presented.
Table 1: Descriptive statistics
Figure 2: Measurement of transcerebellar diameter of 32.1 mm at 29.1 weeks of gestation
Figure 3: Measurement of transcerebellar diameter of 28.6 mm at 27 weeks of gestation
[Table 2] shows the relationship between TCD with other biometrics and period of gestation in normal pregnancy. The cases were divided into 4 groups. The mean TCD at 27 weeks was 30.073 ± standard deviation (SD) 0.863, at 28 weeks it was 30.838 ± SD 0.727 mm, at 29 weeks it was 31.783 ± SD 0.787 mm and raised up to 32.5 ± SD 0.973 mm at 30 weeks of gestation.
Table 2: Mean and standard deviation for biparietal diameter, abdominal circumference, head circumference, femoral length, and transcerebellar of gestational age subgroups based on period of gestation
When individual observations of mean TCD in normal pregnancy cases were studied in relation to the period of gestation in weeks, linear relationship seen between the period of gestation and TCD. The mean observations of other standard biometrics are tabulated in [Table 2]. The correlation coefficient between period of gestation and TCD was found to be 0.99 which was statistically significant (P < 0.001) and other standard biometrics were found a better correlation with the GA [Table 3]. [Figure 4] shown in the scatter diagram, there is a good linear relationship seen between GA and TCD.
Table 3: Correlation coefficient with third-trimester gestational age
Figure 4: Linear relationship seen between gestational age and transcerebellar diameter -Figure showing the transcerebellar diameter increased linearly against the gestational age increases
Out of 100 mothers, 57 were primigravida and 43 were multigravida status, and there is no difference between primigravida and multigravida in terms of gestational weeks [Table 4].
Table 4: Distribution of pregnancy by gravidity versus gestational weeks
Linear regression models for estimation of GA were derived from all the biometric indices (TCD, BPD, FL, and AC). In addition, stepwise regression models were constructed to determine the best model for the determination of GA between 27 and 30 weeks of gestation. Comparison of the accuracy of these models in the determination of GA showed that TCD has 99% predictive accuracy, with a standard error of 1.6 days. In this study, the regression coefficient of the transcerebral diameter was +0.48 with a standard error of 0.053 and P < 0.01 [Table 5]. [Figure 5] shown in the scatter diagram, the linear regression seen between GA and TCD.
Table 5: Linear regression coefficient of transcerebellar against gestational age
Figure 5: Linear regression of transcerebellar diameter against gestational age-showing in the scatter diagram, the linear regression seen between gestational age and transcerebellar diameter at the 95.0% confidence level
Discussion
Accurate gestational dating is of paramount importance and the cornerstone for the management of pregnancies; easily reproducible sonographic fetal biometric parameters for gestational dating are clinically important for the best obstetric management of pregnancies. This is especially true in determining the timing of a variety of gestational tests, assessing the adequacy of growth and timing of delivery for the optimal obstetric outcome. Campbell et al. demonstrated that 45% of pregnant women are uncertain of menstrual dates as a result of poor recall, irregular cycles, bleeding in early pregnancy, or oral contraceptive use within 2 months of conception.
The most reliable parameter used in the estimation of the GA in the second and early third trimester is the BPD; femur length (FL) is the most accurate for the late third trimester. The measurement of BPD in the second-trimester routine scan is performed in all good antenatal care centers.
As per ultrasound literature TCD is a unique, reliable parameter for estimating the duration of gestation and is consistently superior in predicting GA in both singleton and twin gestation.[1112] The TCD has been shown to be a reliable parameter that is significantly correlated with GA by the end of the second trimester.[4]
Prediction of GA in the pregnancies without precise last menstrual period information is a tough task for sonologists, especially in case of growth-restricted pregnancies, although in normal pregnancy fetal biometric parameters are helpful for the estimation of GA as successive growth of these parameters occurs during the course of pregnancy. Hence, the focus of a sonologist is generally on such parameters that can be used independently without being affected by the impaired growth of the fetus. In recent years, several studies revealed that TCD has been identified as a useful growth indicator that stands the growth pattern irrespective of the overall growth pattern of pregnancy. Routine biometric parameters for GA assessment such as BPD, HC, AC, and FL have their own limitations like BPD and HC are difficult to measure because of molding of the head in the third trimester. Similarly, femur length is not reliable in cases of achondroplasia.
In a study done on TCD in singleton pregnancies by Gupta AD et al[13] from India, the authors observed that the GA of pregnant women not sure of their LMP can be reliably estimated by measuring the TCD which showed good correlation (r = +0.946, r2 = 89.6% and P < 0.001). The increase in TCD throughout gestation helped in assessing the development of the cerebellum.[14] The present study is an attempt to find whether fetal TCD can be used as an independent parameter to calculate the GA like the other established biometric indices (BPD, HC, FL, AC) and accuracy to which GA can be calculated if TCD is added to the four routine biometric parameters. An often quoted rule of thumb is that “TCD in mm approximates GA in weeks.”
The increase in TCD throughout gestation helped in calculating the development of the cerebellum. Guan B[15] found curvilinear relationship between TCD and GA (R2 = 0.99624, P < 0.0005). He inferred that the function of the TCD in the evaluation of fetal growth and development is better than any other biometrics. Mikovic and Markovic et al.[16] studied the growth of fetal cerebellum in normal pregnant mothers between 20 and 40 gestational weeks and proposed that TCD can be practically applied in cases where it is difficult or impossible to measure BPD or in cases where it is unsuitable because of the expressed molding of the head.
When individual observation of mean TCD was studied in relation to the period of gestation in weeks [Table 3], the correlation coefficient was found to be 0.99, which was statistically significant (P < 0.001), similar to the findings of Meyer WJ et al.,[13] Goel P. et al.[17] Furthermore, it was deduced that the other standard biometric values had a good correlation with GA [Table 2] and established a better correlation coefficient as shown in Table 3.
From the regression analysis, a strong relationship has been observed between fetal TCD and GA. With increasing GA, there was a significant increase in TCD values. The correlation between TCD measurements and GA was strong at 27–30 weeks (r > 0.99). Using correlation coefficients, it has been predicted that for every one mm increase in TCD there is an increase of 0.4 weeks in GA, i. e., one day. There is a statistically significant relationship between TCD and GA at the 95.0% confidence level.
Conclusions
This study showed that TCD is an accurate predictor of GA in the third trimester. The correlation between GA and the GA by TCD seems to increase from 28 to 30 weeks. From the present study, it was observed that the TCD increases linearly with GA. There is a good correlation between GA derived from TCD and GA from established biometric indices like BPD, HC, AC, and FL. The TCD in mm, and this parameter alone can predict the GA, TCD can be combined with the other four biometric indices to give a fair estimation of GA. Hopefully, the results will demonstrate its applicability in routine practice.
Consent
Written informed consent was taken from the patients for publication of this research study.
Approval from IRB: Yes
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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