Abstract
Background
Ultrasonographic estimation of gestational age is done routinely by using fetal biometric parameters, such as the biparietal diameter (BPD), femur length (FL), abdominal circumference (AC), and head circumference (HC). Gestational dating by ultrasound is reliable in the 1st trimester, but discrepancies increase to more than 3 weeks in the 3rd trimester.
Aims and objectives
To study the accuracy of gestational age estimation by the transcerebellar diameter (TCD) in the 3rd trimester of singleton pregnancies whose gestational age is known.
Materials and methods
A prospective study was carried out in the Department of Obstetrics and Gynecology at NDMC Medical College and Hindu Rao Hospital, Delhi, India, from October 2017 to May 2019 after obtaining clearance from the ethical committee. The study included 100 women aged 18–35 years with singleton pregnancies in the 3rd trimester with a gestational age ≥ 28 weeks; the women provided informed consent and filled in an F-form to rule out any sex determination of the baby. Only those women who had regular menstrual cycles 6 months before conception, who were sure of their last menstrual period (LMP), who had a gestational dating scan done up to 14 weeks, and for whom congenital malformation of the fetus had been ruled out by mid-trimester ultrasound were included. Gestational age was obtained for the measured BPD, HC, AC, FL, and TCD. A paired t-test was used for the comparison between the period of gestation (POG) by the 1st ultrasonography (USG) and that by the AC, BPD, FL, HC, and TCD.
Results
The estimation of gestational age by the BPD becomes gradually less reliable after 32 weeks of pregnancy, that by the HC and FL after 36 weeks, and that by the AC after 28 weeks. Regression analysis was used to find a correlation of the various parameters, such as the POG by the 1st USG, BPD, HC, AC, FL, and TCD, with the POG by LMP. The correlation coefficient (r), coefficient of determination (R2), and p values were calculated. Of all the sonographic parameters for the estimation of gestational age, the TCD showed the highest correlation (r = 0.979; p < 0.0001) and the AC the lowest correlation.
Conclusion
The mean difference between the estimate of gestational age by the TCD and that by the 1st trimester USG increased from ± 1 day at weeks 28–32 to ± 1–2 days at weeks 32–36 and further increased to ± 6 days after 36 weeks of pregnancy, which was statistically significant. This indicated that the TCD could estimate gestational age within 6 days of near-term pregnancy. Thus, the TCD was a reliable parameter for the estimation of gestational age within 6 days of near-term pregnancy along with routine biometry and an alternative parameter for the evaluation of gestational age when the LMP is not known.
Keywords: Gestational age estimation, Ultrasound, Transcerebellar diameter (TCD), Fetal biometry
Introduction
It is important to have an accurate estimate of the gestational age (GA) and expected date of delivery for formulating management regarding the time of delivery, either in the form of planned induction of labor or a Cesaerean section [1, 2]. Gestational dating also helps in distinguishing the low birth weight of babies due to prematurity or growth restrictions in the 3rd trimester [3]. The last menstrual period (LMP) is the most common method used to date pregnancy if the woman is sure of her exact dates [4], but it cannot be relied upon because of faulty memory, irregular cycles, variation in ovulation date, conception in lactational amenorrhoea, or bleeding in the 1st trimester [5, 6]. Ultrasonographic estimation of GA is done routinely by measuring fetal biometric parameters, such as the biparietal diameter (BPD), femur length (FL), abdominal circumference (AC), and head circumference (HC). Gestational dating by ultrasound is reliable in the 1st trimester, but discrepancies increase to more than 3 weeks in the 3rd trimester [7]. The BPD depends on fetal growth and the shape of the fetal skull, and the HC is comparable to the BPD. AC values vary according to fetal growth, with higher values in fetal macrosomia and lower values in fetal growth retardation. The FL is affected by artefactual bowing of the legs or inclusion of the distal epiphysis, which influences the measurement of the FL in the 3rd trimester [8–12]. The fetal transcerebellar diameter (TCD) is one of the sonographic parameters that has evoked the interest of researchers for its ability to estimate GA [13]. Unlike the AC, the TCD is little affected in cases of fetal growth restrictions; hence, the GA can be predicted accurately with the TCD even in the 3rd trimester. In India, being a developing country, many women come for the first time to the hospital in the 3rd trimester and do not remember their LMP well, so it is difficult to estimate the GA of the fetus. With the advantages of the TCD over other fetal biometric parameters, we evaluated the accuracy of TCD measurements for the estimation of GA and compared it with other ultrasound parameters used routinely for gestational dating, including the BPD, HC, AC, and FL, in the 3rd trimester of normal singleton pregnancies in the Indian population whose gestational age was known by early pregnancy gestational dating scans of up to 14 weeks [13].
Material and methods
Study setting
This prospective study was carried out in the Department of Obstetrics and Gynecology at NDMC Medical College and Hindu Rao Hospital, Delhi, India, from October 2017 to May 2019. Due clearance from the ethical committee of our hospital was obtained for this study.
Study procedure
The study population included 100 pregnant women between 18 and 35 years old who attended the hospital with singleton pregnancies in the 3rd trimester with a GA ≥ 28 weeks.
Consent was taken as per inclusion and exclusion criteria. Only those women who had regular menstrual cycles 6 months before conception, who were sure of their LMP, who had a gestational dating scan done up to 14 weeks, and for whom congenital malformation of the fetus had been ruled out by mid-trimester ultrasound were included after obtaining informed consent.
Excluded were women with the following conditions:
Bleeding in the 1st trimester or a history of abnormal babies, stillbirths, or intrauterine death (IUDs).
Medical comorbidities, such as heart disease, diabetes mellitus, gestational diabetes mellitus, chronic hypertension, gestational hypertension, thyroid disorders, and anti-phospholipid antibody syndrome.
Oligohydramnios, polyhydramnios, and intrauterine growth restriction (IUGR).
A difference of 2–3 weeks between the LMP and the early pregnancy gestational dating scan.
After the relevant history was taken, a physical examination was done to rule out the exclusion criteria. Ultrasound was performed after the women provided written consent and filled out an F-form as per government guidelines. Ultrasound was done using a MINDRAY ultrasound scanner with a 3.5-MHz curvilinear probe, with the pregnant women in the supine position. The fetal lie, BPD, HC, AC, FL were measured using standard methodology. The TCD was obtained in the axial plane in the cerebellar view, i.e., with a slight rotation of the transducer of approximately 30° from the conventional thalamic plane, while the BPD was measured using the cavum septum pellucidum, third ventricle, and thalami as landmarks. In this plane, the posterior fossa of the cerebellum, with its characteristic butterfly-like appearance of two lobules, was visualized on either side of the midline. The cisterna magna is just posterior to the cerebellum. This plane provides the widest TCD, which was measured by placing the electronic calipers from the proximal outer to the distal outer margins of the cerebellum.
GA was obtained for the measured BPD, HC, AC, FL, and TCD (14–56 mm) in weeks, as determined by the built-in GA prediction software in the MINDRAY ultrasound scanner according to the nomograms of Hadlock et al. [10].
Statistical analysis and results
The data were entered in an MS Excel sheet and analysed using SPSS version 21.0. A paired t-test was used for the comparison between the period of gestation (POG) by the 1st ultrasonography (USG) and that by the AC, BPD, FL, HC, and TCD. A chi-square test was used to correlate the qualitative variables. Pearson’s correlation coefficient was used to assess the association of the various feral biometric parameters with the POG by LMP or ultrasound in early pregnancy.
Univariate linear regression was used to determine the predictability of the TCD to assess the significant predictors of the POG by LMP. A p value < 0.05 was considered statistically significant.
The majority of the women were between 21 and 30 years old. Of the 100 pregnant women, 77 were primigravida, and 23 were multigravida. The majority of the primigravida (48%) pregnancies were between 32 and 36 weeks, and 43% of the multigravida pregnancies were between 36 and 40 weeks. Age and parity were comparable in all the groups. A total of 89 women had a cephalic presentation, 9 women had a breech presentation, and 2 women had a transverse lie (Table 1).
Table 1.
Estimation of the POG by the 1st trimester USG and the POG by the BPD, HC, AC, FL, and TCD in the 3rd trimester and the average TCD in the 3rd trimester
| GA (weeks) | No. of patients | Mean POG by 1st trimester USG | Mean GA by USG (mean ± days) | |||||
|---|---|---|---|---|---|---|---|---|
| BPD ± days | HC | AC | FL | TCD | Average TCD (mm) | |||
| 28–32 | 29 | 30.57 ± 0.97 | 30.64 ± 2 | 30.55 ± 2 | 29.95 ± 2–7 | 30.53 ± 1–4 | 30.46 ± 1 | 35 ± 0.98 |
| p value | 0.861 | 0.686 | 0.001 | 0.002 | 0.116 | |||
| 32–36 | 46 | 34.23 ± 1.09 | 33.89 ± 2–4 | 33.94 ± 2–4 | 32.92 ± 7–11 | 34.02 ± 2–7 | 34.01 ± 1–2 | 40 ± 1.04 |
| p value | 0.025 | 0.074 | < .0001 | 0.636 | 0.022 | |||
| 36–40 | 25 | 37.17 ± 0.97 | 36.37 ± 2–9 | 36.38 ± 9 | 35.33 ± 11–15 | 36.84 ± 4–10 | 36.27 ± 6 | 43 ± 0.88 |
| p value | 0.003 | 0.028 | < 0.0001 | 0.0001 | < 0.0001 | |||
All the fetal biometric parameters increase as the baby grows with advancing GA. The average TCD in the 3rd trimester was found to be 35 ± 0.98 mm at weeks 28–32, 40 ± 1.04 mm at weeks 32–36, and 43 ± 0.88 at weeks 36–40 (Fig. 1).
Fig. 1.
Scatter diagrams showing a linear correlation between the GA by the 1st trimester USG and the estimated GA by the BPD (a), HC (b), AC (c), FL (d), and TCD (e)
The mean difference in the GA estimate by the TCD and other routine biometric parameters (BPD, HC, AC, and FL) increased gradually from week 28 to week 40 in the 3rd trimester (Table 2). We observed the following mean differences between the GA estimate based on the 1st trimester USG and that based on the biometric parameters:
The BPD is ± 2 days at weeks 28–32 and ± 2–4 days at weeks 32–36 and increases to ± 2–9 days after 36 weeks of pregnancy. This difference is statistically significant after 32 weeks.
The HC is ± 2 days at weeks 28–32 and ± 2–4 days at weeks 32–36 and increases to ± 9 days after 36 weeks of pregnancy. This difference is statistically significant after 36 weeks.
The AC varies from ± 2–7 days at weeks 28–32 to ± 7–11 days at weeks 32–36 and further increases to ± 11–15 days after 36 weeks of pregnancy. This difference is statistically significant throughout the whole 3rd trimester (after 28 weeks).
The FL is ± 1–4 days at weeks 28–32 and ± 2–7 days at weeks 32–36 and further increases to ± 4–10 days after 36 weeks of pregnancy. This difference is statistically significant in the 3rd trimester.
The TCD is ± 1 day at weeks 28–32, ± 1–2 days at weeks 32–36, and within 6 days of the POG by LMP after 36 weeks of pregnancy.
Table 2.
Univariate linear regression and correlation of the POG by the 1st trimester USG with different USG parameters for estimating GA in the 3rd trimester (Pearson’s correlation coefficient, r)
| POG by 1st trimester USG vs | Pearson’s correlation coefficient (r) | Coefficient of determination (R2) % | p value |
|---|---|---|---|
| BPD | 0.928 | 86.59 | < 0.0001 |
| HC | 0.937 | 89.19 | < 0.0001 |
| AC | 0.915 | 84.81 | < 0.0001 |
| FL | 0.965 | 94.35 | < 0.0001 |
| TCD | 0.971 | 95.84 | < 0.0001 |
Thereby, the GA estimate based on the BPD becomes gradually less reliable after 32 weeks of pregnancy, the one based on the HC and FL after 36 weeks, and the one based on the AC after 28 weeks. Table 2 and Fig. 2 show increased scatter in the GA values calculated by the BPD, HC, AC, FL, and TCD as compared with the 1st trimester USG.
Fig. 2.
Bar diagram for the correlation of the POG by the 1st trimester USG with that by the BPD, HC, AC, FL, and TCD
As the reliability of these parameters decreases towards term pregnancy, regression analysis was done, and the correlation coefficient (r), coefficient of determination (R2), and p-values were calculated (Table 2, Fig. 2). Pearson’s correlation coefficient (r) was used to compare the correlation between the various sonographic parameters, such as the BPD, HC, AC, FL, and TCD, and the POG by the 1st trimester USG.
Our study showed good correlation between the GA of the fetus and all the above sonographic parameters. This correlation with the POG by the 1st trimester USG was maximum for the TCD (r = 0.971), followed by the FL (r = 0.965), HC (r = 0.937), and BPD (r = 0.928), and was minimum for the AC (r = 0.915). Of all the sonographic parameters for the estimation of GA, the TCD showed the highest correlation (r = 0.979; p < 0.0001) and the AC the lowest correlation. This correlation is statistically significant with a p value < 0.0001.
The predictive accuracy of the POG by LMP compared with the other parameters is shown by the value of R2. Of all the sonographic parameters, the predictive accuracy of LMP was the highest for the TCD, with R2 = 95.84% (p < 0.0001). This indicates that the TCD is a reliable and independent parameter for the estimation of GA even in the 3rd trimester.
The advantage of this linear regression analysis is that we can calculate the LMP if we know the TCD in the 3rd trimester. The POG by LMP (weeks) = − 2.211 + 1.076 * TCD (weeks); thus, if the TCD increases by 1 week, the POG by LMP increases by 1.076 weeks.
Discussion
Many pregnant women come to the hospital in the 3rd trimester with no recollection of their LMP. This makes it difficult for the clinician to decide whether the pregnancy is preterm or growth restricted. In such circumstances, calculating GA by ultrasound using the BPD, AC, or HC is inaccurate and has a discrepancy of more than 3 weeks. Furthermore, an independent sonographic parameter is required for an accurate GA estimate. The fetal cerebellum can be visualized by ultrasound at weeks 10–11. Moreover, the fetal cerebellum grows steadily with advancing GA and has a linear correlation with it. The cerebellum is located in the posterior cranial fossa and is surrounded by dense petrous bone and occipital bone. Hence, the fetal cerebellum does not vary with a change in the shape of the fetal skull. The TCD is not affected by fetal growth and is hardly affected even in cases of severe IUGR. These features make the TCD one of the most reliable ultrasound parameters for assessing GA even in the 3rd trimester [14]. In 2011, Capponi et al. described a novel algorithm based on innovative display technology for visualizing the sagittal and coronal planes of the fetal brain by three-dimensional (3D) ultrasonography for the examination and diagnosis of anomalies in the fetal brain. The TCD was measured by visualizing the occipital horns of the lateral ventricles and the cerebellar hemispheres and vermis in the coronal plane [15].
Our study showed the highest correlation between the GA estimate by LMP and that by the TCD (r = 0.979), followed by the FL (r = 0.971), HC (r = 0.944), and BPD (r = 0.931), and the lowest correlation for the AC (r = 0.921) in the 3rd trimester, which is statistically significant with a p value < 0.0001. These results are similar to those of previous studies, such as Prasad et al. [16], Adeyekun et al. [17], and Jayaprakash et al. [18], in which the TCD showed the best correlation with LMP when compared with other routine ultrasonographic biometric parameters (BPD, HC, AC, and FL). This indicates that the TCD is a reliable parameter for the estimation of GA even in the 3rd trimester (Table 3).
Table 3.
Comparison between the present study and other studies of the correlation of GA estimation by the POG by LMP with that by the BPD, HC, AC, FL, and TCD in the 3rd trimester
| Study | GA (weeks) | Correlation coefficient (r) for POG by LMP vs | ||||
|---|---|---|---|---|---|---|
| BPD | HC | AC | FL | TCD | ||
| Present study | 28–40 | 0.931 | 0.944 | 0.921 | 0.971 | 0.979 |
| Prasad et al. (2016) [16] | 15–40 | 0.964 | 0.982 | 0.965 | 0.983 | 0.989 |
| Adeyekun et al. (2014) [17] | 14–42 | 0.969 | 0.963 | 0.974 | 0.984 | |
| Jayaprakash et al. (2018) [18] | 28–40 | 0.937 | 0.947 | 0.952 | 0.960 | 0.960 |
Our study showed the highest predictive accuracy of GA by the TCD as compared with other sonographic biometric parameters in the 3rd trimester, as the TCD has the highest R2 value. This result is similar to that of a previous study by Prasad et al. [16]. However, Mona Al et al. [19] found the highest predictive accuracy for the BPD, with values of the TCD comparable to those of the FL and HC (Table 4).
Table 4.
Comparison between the present study and other studies of the predictive accuracy of GA estimation by the BPD, HC, AC, FL, and TCD in the 3rd trimester
We found a positive correlation between the GA estimate by the TCD and that by the LMP in the 3rd trimester (r = 0.979; R2 = 95.84%), which is statistically significant with a p value < 0.0001. The strength of the correlation was larger than 0.95 in all the previous studies, as shown in Table 5. The predictive accuracy of the TCD for GA estimation in the 3rd trimester was almost 95% in most of the studies, except those by Gupta et al. [22] and Mona Al et al. [19], in which the accuracy was only 89.6% and 86%, respectively (Table 5).
Table 5.
Comparison between the present study and other studies of the correlation of GA estimation by the LMP with that by the TCD in the 3rd trimester
| Study | Sample size | GA (weeks) | Correlation coefficient (r) | Coefficient of determination (R2) (%) | p value |
|---|---|---|---|---|---|
| Present study | 100 | 28–40 | 0.979 | 95.84 | < 0.001 |
| Prasad et al. [16] | 60 | 15–40 | 0.989 | 97.8 | < 0.001 |
| Uikey et al. [20] | 301 | 14–40 | 0.972 | 98.56 | < 0.001 |
| Ravindernath et al. [21] | 100 | 15–40 | 0.998 | 99.5 | < 0.0001 |
| Mona Al et al. [19] | 330 | 14–40 | 86 | 0.000 |
In the present study, the mean TCD was 35 ± 0.98, 40 ± 1.04, and 43 ± 0.88 mm at weeks 28–32, 32–36, and 36–40, respectively. The mean difference between the GA estimate by the TCD and that by the 1st trimester USG increases from ± 1 day at weeks 28–32 to ± 1–2 days at weeks 32–36 and further increases to ± 6 days after 36 weeks of pregnancy, which is statistically significant in the 3rd trimester after 32 weeks, indicating that the TCD can estimate GA within 6 days of near-term pregnancy.
This result is similar to that of a study by Chavez (2004), in which the difference between the actual GA and the predicted GA by the TCD ranged from 0–4 days at weeks 17–21 to 0–2 days at weeks 22–28 of gestation in the 2nd trimester, and from 0–5 days at weeks 29–36 to 0–9 days at week 37 of gestation in the 3rd trimester [23]; in Prasad et al. [16], it was 1.1 weeks.
The result is different from that of a study by Chavez (2003), in which the predicted GA by the TCD differed in the 3rd trimester by 1–2 days at weeks 28–30 and by 4–6 days after 32 weeks [24]; in Davies et al. (2001), it was ± 2.33 weeks [25].
Univariate linear regression
The linear regression analysis was used to calculate the LMP if we know the TCD. Notably, the linear model equations produced by the regression analysis, and shown below, demonstrate the relationship between the TCD and the POG by LMP, as obtained in various studies:
Present study: POG by LMP (weeks) = -2.211 + 1.076 * TCD (weeks).
Dashottar et al. [26]: GA (weeks) = 0.547 * TCD + 9.231 (weeks).
Eze et al. [27], with GA (Y) in weeks and TCD in millimeters: Y = 0.63 (TCD) + 7.16,
Y = 0.54 (TCD) + 14.32, and Y = 0.79 (TCD) + 4.91 in the 2nd, 3rd, and combined trimesters, respectively.
These equations are different in various studies, as the sample size is small. Therefore, studies with a larger sample size are required to derive a standard equation for the calculation of GA.
Conclusions
The mean TCD was 35 ± 0.98, 40 ± 1.04, and 43 ± 0.88 mm at weeks 28–32, 32–36, and 36–40, respectively. The mean difference between the GA estimate by the TCD and that by the 1st trimester USG increased from ± 1 day at weeks 28–32 to ± 1–2 days at weeks 32–36 and further increased to ± 6 days after 36 weeks of pregnancy, which was statistically significant. This indicated that the TCD could estimate GA within 6 days of near-term pregnancy. The GA estimate by LMP has the best correlation with the TCD as compared with the BPD, HC, AC, and FL after 28 weeks of pregnancy. Of all the fetal biometric parameters, the TCD has the highest predictive accuracy for estimating GA in the 3rd trimester. We conclude that the TCD was a reliable parameter for GA estimation within 6 days of near-term pregnancy along with other routine biometric and other ultrasound parameters for evaluating GA in cases in which the LMP is unknown.
Compliance with ethical standards
Conflict of interest
There is no conflict of interest.
Ethical statements
All human studies have been approved by the ethical committee of our hospital. So the study has been performed in accordance with the ethical standards laid down in Helseinki Declaration of 1975 and its late amendments.
Informed consent
Additional informed consent was obtained from all the patients for which identifying information is not included in this article.
Footnotes
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Contributor Information
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