Comparative Accuracy of Trans Cerebellar Diameter and Crown Rump Length for Estimation of Gestational Age

Many Indian women present for the first antenatal check up in third trimester and most of them have unreliable LMP with no early scans. To compare the predicted accuracy of third trimester transcerebellar diameter and first trimester crown rump length for estimation of gestational age. A cross sectional study was conducted on 198 eligible antenatal women in 3 rd trimester, having early scans showing CRL. Transcerebellar diameter (TCD) was measured and compared with 1 st trimester CRL for accurately estimating the gestational age based on LMP. It was found that there is linear rise in TCD along with gestational age and TCD in mm corresponded to the gestational age in weeks. TCD was accurate in 90.6% women between 29 to 31 weeks and then accuracy was increased reaching 94.7% (33+1 to 35 wks) and 93.8% (37+ to 40 wks). The efficacy of 3 rd trimester TCD is comparable to 1 st trimester CRL for estimation of gestational age.


Introduction
The accurate estimation of gestational age is of great clinical importance to the obstetricians. Some women are not sure of their dates or their LMP cannot be taken into consideration as they may not have regular cycles or had spotting in early weeks or recent usage of hormonal contraceptives. Clinical examination may be incorrect and can be affected by amount of liquor, fetal growth restriction or macrosomia, maternal obesity etc. [1] All women may not have first trimester scan which has good gestational accuracy and fetal biometry in second & third trimester have significant discrepancy.
Wrong estimation of gestational age can lead to unnecessary induction of labour, dysfunctional labour, caesarean section, increased maternal and neonatal morbidity. Unnecessary preterm inductions can also cause neonatal mortality due to consequences of prematurity.
So there is a need of ultrasonographic parameter that can reliably estimate the gestational age at any period and is not altered by any factors. The transcerebellar diameter (TCD) has been found to be a reliable ultrasound parameter for gestational age estimation by the end of second trimester [2]. Cerebellar growth is not altered in fetal growth restriction and its rate of growth is similar in both singleton or multiple gestation [3]. Fetal cerebellum can be visualized sonographically at 9-10 week, it grows rapidly in second trimester having linear relationship with gestational age. Gestational age inweeks corresponds to TCD in mm [4]. Chavez MR devised TCD normograms and reported that it has 94% predictive accuracy for gestational age in third trimester [5]. Predicted gestational age by TCD is within 5 days for GA 29 -36 weeks and at 37 weeks is ± 9 days.
A study was conducted to explore if third trimester TCD is as accurate as first trimester CRL?

Aim
To compare the predicted accuracy of third trimester transcerebellar diameter and first trimester crown rump length for estimation of gestational age.

Material and Methods
A cross sectional study was conducted on antenatal women of any age and parity in third trimester (29 -40 weeks) attending OPD for regular check up and satisfying inclusion criteria. Prior ethical clearance was taken from Institutional ethical committee. Gestational age was calculated from confirmed LMP and CRL, those having discrepancy of more than 2 weeks were excluded.

2) Multiple Gestation / Malpresentation / Previous Caesarean Section
Transcerebellar diameter was measured and compared with CRL (measured in first trimester) for estimation of gestational age. TCD was measured by transverse view of fetal intracranium through the posterior fossa that included visualization of midline thalamus, cerebellar hemisphere and cistern magna ( Figure. 1, 2, 3). Measurements were taken in mm at 90 degree to the long axis of the cerebellum across its widest point using outer to outer method [5]. TCD and CRL were considered accurate if the discrepancy between them and LMP was within 5 days.    TCD can be classified into three grades: Grade 1 -is seen up to 27 weeks, cerebellar hemisphere is rounded with absence of echogenicity and poorly developed vermis that gives eyeglass appearance to cerebellum. Grade 2 -is seen from 28 wks to 32 weeks, vermis is prominent and appears as echogenic rectangular tissue and cerebellum has "dumbbell appearance". Grade 3 -is seen after 32 weeks gestation, hemispheres become triangular or fan shaped and cerebellum Richa Sharma and Natasha Gupta: Comparative Accuracy of Trans Cerebellar Diameter and Crown Rump Length for Estimation of Gestational Age looks more solid than cystic. Data collected was entered in Statistical Package for social sciences (SPSS) software. Percentage was calculated and chisquare test was applied. P-value 0.005 was taken as significant.
There was linear rise in TCD along with gestational age and TCD in mm corresponded to the gestational age in weeks. TCD was accurate in 90.6% women and CRL was accurate in 93.7% women between 29 to 31 weeks, which is statistically not significant p=0.2. TCD accuracy was there after increased reaching 94.7% (33+1 to 35 wks) and 93.8% (37+ to 40 wks). [ Table-2].

Discussion
Many antenatal women in India report for their first antenatal check up in third trimester with unreliable last menstrual period and no early scan. Decision for induction of labour for any obstetric indication becomes difficult in these women. Various studies reported the better correlation of TCD with gestational age in second and third trimester, its usefulness as growth assessing parameter in comparison with other routine ultrasound parameters. In our study, CRL was used for comparison with TCD. The CRL is most commonly used ultrasound parameter and standard method for gestational age assessment in first trimester which is accurate, and thus compared.
Transcerebellar diameter is an independent ultrasonographic parameter for estimation of gestational age, which could be as reliable as first trimester CRL as depicted in our study.
Fetal cerebellum does not change its form and size due to dense surrounding petrous ridges and occipital bone [7].
Use of TCD between 17 to 40 weeks was studied by Reece et al, they reported it an accurate indicator of gestational age [8]. Malik et al observed TCD and BPD, FL, & AC in 135 women between 26 to 38 weeks. Reported that gestational age measured by TCD was consistently correlated with that measured by BPD, FL & AC. They also found that cerebella in 46%, 25%, 29% were grade I to III respectively, that changed progressively with advancing gestation. The median GA and TCD were 20 wks and 21mm for grade I, 31 wks and 36 mm for grade II and 36 wks and 42 mm for grade III [9]. Martin R et al studied TCD in FGR and large fetuses and reported that predicted gestational age in FGR was within 3 days in 97.5% in 2 nd trimester and 93.3% in 3 rd trimester and in large fetuses it was 100% within 3 days in 2 nd & 3 rd trimesters. Concordance between actual and predicted GA for FGR fetuses using the singleton TCD normogram was high (Pearson's correlation, r = 0.98; P<0.001). Pearson's correlation coefficient for concordance between actual and predicted GA's in large fetuses was 0.95 (P< 0.001) [10]. Naseem et al assessed 228 women at 36 weeks and reported that the accuracy of TCD was 91.7% compared to BPD 77.2%. When they compared the proportion of correct assessment by these parameters was compared by chi-square test, value of chi-square was found to be 10.472a. P value was 0.001, and found to be statistically significant proving the hypothesis that TCD is more reliable method of gestational age estimation in third trimester [11].
To best of our knowledge, Present study is the first one to compare the accuracy of 3 rd trimester TCD and 1 st trimester CRL.

Conclusion
The efficacy of TCD is comparable to CRL (where there may be discrepancy of 5 day) and this knowledge can be utilized with certainty for women with unknown LMP and non availability of early dating scans.