Accurate gestational dating is one of the most important assessments that obstetrical providers perform in pregnancy, given that all of the various management strategies are dependent on knowing where the patient is in gestation. In addition to traditional biometry, ancillary biometric and non biometric measurements can help narrow the biologic variability between fetuses. Moreover, one can use these nontraditional measurements both in late gestation to help determine appropriate gestational age (GA) and fetal lung maturity. These methods can also be used for specific clinical situations, such as oligohydramnios, in which compression of the fetal head and abdomen can lead to difficulty in obtaining an accurate biparietal diameter (BPD) and abdominal circumference 1.
The estimation of pregnancy dates is important for the mother and her healthcare provider. The three basic methods used to estimate GA are menstrual history, clinical examination, and ultrasonography 2. Since the beginning of ultrasound fetal measurements, the possibility of differences in populations has been considered 3. Some researchers have suggested that population differences in fetal biometry are negligible and that separate standards are not essential 4.
Naegle’s rule is the most common method of pregnancy dating. The expected date of delivery is calculated by counting back 3 months from the menstrual period and adding 7 days. Inaccuracy occurs because many women do not have regular 28-day cycles or conceive on day 14, and many others are not certain of the date of their last period. In addition, early pregnancy bleeding or recent use of hormonal contraceptives may lead to an incorrect assumption of the date of ovulation 5.
The transverse cerebellar diameter (TCD) has been one of the most reliable ultrasound parameters for growth, especially early gestation 6. The fetal cerebellum shows progressive growth throughout the gestation period 5,7; thus, it is an organ capable of providing information on the prediction of GA during the pregnancy. The TCD is well established in the ultrasound literature as a reliable parameter for estimating GA. This parameter may be particularly useful for accurate dating of pregnancies in the third trimester 8.
The TCD serves as a valid indicator of GA in the fetus and is a standard against which aberrations in other fetal parameters may be compared, especially when the GA cannot be determined by the routine methods of an early pregnancy scan or the date of the last menstrual period 7.
Patients and methods
This study was carried out on 150 women with normally progressing pregnancies during the second and third trimesters. The women were attending the outpatient clinic of the obstetrics and Gynaecology Department, Beni-Sueif Faculty of Medicine, in the period from March 2012 to March 2013. All participants signed an informed consent. The criteria for inclusion in the study were singleton gestation, head presentation, absence of fetal anomalies, no history of chronic diseases: diabetes and/or hypertension, and no history of abnormal babies, or stillbirths. Participants with multiple pregnancies, congenital fetal malformations, IUGR, and fetal macrosomia were excluded.
All patients were subjected to a thorough assessment of history, with a focus on the date of the last menstrual period to ensure its reliability, general and abdominal examination, and ultrasound examination to assess fetal viability, BPD, TCD, and femur length (FL).
Fetal TCD was measured using the method described by Goldstein et al. 9 to locate the cerebellum in the posterior fossa by means of rotation of the transducer to ∼30° from the plane that identifies the thalamus, the cavity of the septum pellucidum, the third ventricle, and the cisterna magna, positioning the calipers on the outer margins of the cerebellar hemispheres.
A single measurement was used for each patient. For fetuses that had multiple sonographic examinations at different GAs that included TCD measurements, only the last TCD examination was used in the construction of the nomogram.
The data collected were converted into variables that were analyzed using SPSS version 15 (Illinois, Chicago, USA). Descriptive statistics included mean, SD, and range, which were computed for TCD and other fetal biometric parameters.
The percentiles of GA were calculated. A percentile is the value of a variable below which a certain percent of observations fall. Thus, the 90th percentile is the value below which 90% of the observations may be found.
Comparison of actual and predicted GAs was performed using a paired t-test. Concordance between predicted and actual GA was assessed on the basis of a calculation of the Pearson linear correlation coefficient. Further estimation of the difference between the actual and the predicted GAs on the basis of the TCD measurements and its accuracy was performed.
For comparative statistical analysis and for Pearson linear correlation, the calculated two-tailed P-value less than 0.05 was considered statistically significant and a P-value more than this value was considered statistically insignificant.
The demographic data of the patients were as follows: the mean age of the 150 participants studied was 28.1 years (range, 18–46 years); parity, 2 (range, 0–5); weight, 69.3 kg (range, 45–100 kg); height, 161.9 cm (range, 145–174 cm); BMI, 26.6 (range, 16.41–35); and GA, 29.4 (range, 15–38). 43.3% of the women included had GA between 28 and 34 weeks as estimated from LMP and standard biometric parameters BPD and FL. The mean actual GA was 16.16±0.75 for TCD of 16 mm and it increased up to 37±1.15 for TCD of 42 mm.
For the percentiles of actual GA in relation to TCD measurements, the 5th percentile ranged from 15 weeks for TCD of 16 mm to 36 weeks for TCD of 42 mm. The 50th percentile ranged from 16 weeks for TCD of 16 mm to 37 weeks for TCD of 42 mm. The 90th percentile ranged from 17 weeks for TCD of 16 mm to 38 weeks for TCD of 42 mm. Most of the participants studied (45.3%) had predicted GA between 28 and 34 weeks as estimated by TCD.
The mean predicted GA was 14.83±1.47 for TCD of 16 mm and it increased up to 36.7±0.95 for TCD of 42 mm. The percentiles of predicted GA in relation to TCD measurements were as follows: the 5th percentile ranged from 13 weeks for TCD of 16 mm to 36 weeks for TCD of 42 mm. The 50th percentile ranged from 14.5 weeks for TCD of 16 mm to 36.5 weeks for TCD of 42 mm. The 90th percentile ranged from 16.25 weeks for TCD of 16 mm to 37.75 weeks for TCD of 42 mm. When the actual GA was compared with predict GA by TCD throughout gestation in relation to TCD measurements, there were insignificant differences between the actual and the predicted ages.
The mean difference between BPD-based GA and TCD-based GA was 0.96±1.91 weeks (range, −4 to 4 weeks) and the mean difference between FL-based GA and TCD-based GA was 0.72±1.73 weeks (range, −6 to 5 weeks).
Through the second and third trimesters, there was no difference between the actual and the predicted GA (weeks) in 36% of the participants (N=150). TCD predicts GA up to ±2 week in 83.3% of gestations. Actual to predicted GA difference was ±1 in 23.3%, ±2 in 24%, ±3 in 8%, ±4 in 7.3%, ±5 in 0.7%, and ±6 in 0.7% (Table 1).
Through the second trimester only, there was no difference between the actual and the predicted GA (weeks) in 18.2% of the participants (N=33). TCD predicts GA up to ±2 week in 75.8% of second-trimester gestations. The actual to predicted GA difference was ±1 in 21.2%, ±2 in 36.4%, ±3 in 15.1%, ±4 in 9.1% of the participants (Table 2).
Through the third trimester only, there was no difference between the actual and the predicted GA (weeks) in 41% of the participants (N=117). TCD predicts GA up to ±2 week in 86% of third-trimester gestations. The actual to predicted GA difference was ±1 in 24%, ±2 in 21%, ±3 in 6%, ±4 in 7%, ±5 in 1%, and ±6 in 1% (Table 3).
In terms of the linear correlation of actual GA (weeks) to predicted GA (weeks) by TCD in all pregnancies, there was a significant positive correlation between predicted GA by TCD and actual GA (Pearson’s correlation coefficient=0.965, P<0.0001). The estimated equation of this correlation was as follows: (actual GA=2.13+0.95×predicted GA by TCD) Linear correlation of actual GA (weeks) to predict GA (weeks) by TCD in the second trimester. There was a significant positive correlation between predicted GA by TCD and actual GA (R=0.79, P<0.0001). The estimated equation of this correlation was (actual GA=6.17+0.72×predicted GA by TCD).
In terms of the linear correlation of actual GA (weeks) to predicted GA (weeks) by TCD in the third trimester, there was a significant positive correlation between the predicted GA by TCD and the actual GA (R=0.90, P<0.0001). The estimated equation of this correlation was (actual GA=2.80+0.93×predicted GA by TCD).
Appropriate assessment of GA is paramount in obstetric care. Uncertain GA has been associated with adverse pregnancy outcomes including low birth weight, spontaneous preterm delivery, and perinatal mortality. Making appropriate management decisions and delivering optimal obstetric care requires accurate appraisal of GA 8.
Ultrasound assessment of GA has become an integral part of obstetric practice in recent times 10. Currently, the sonographic estimation is derived from calculations based on fetal measurements and serves as an indirect indicator of GA 8.
Although ultrasound has proven to be useful in the assessment of GA in the first and second trimesters, accuracy in the third trimester is not as reliable. Biological variation can be a major factor that affects accuracy in GA prediction, and this variability increases considerably with advancing pregnancy 8.
The study by Doubilet and Benson 11 evaluated late third-trimester ultrasound examinations of women who had also received a first-trimester examination and found the disparity in GA assessments to be three weeks or greater. However, data from other studies have shown that ultrasound estimation of GA in late pregnancy may be better than indicated in older publications 11.
When choosing the optimal parameter for estimation of GA, it is essential that the structure has little biologic variation, and can be measured with a high degree of reproducibility 12.
In the past, the BPD had been described as a reliable method of determining GA 13. Although the BPD was the first fetal parameter to be clinically utilized in the determination of fetal age in the second trimester, more recent studies have evaluated the use of several other biometric parameters. These parameters include head circumference, abdominal circumference, FL, foot length, and TCD 14.
TCD is a new unique parameter, well established in the ultrasound literature as a reliable parameter for estimating the duration of gestation 6, and it is consistently superior in predicting GA in both singleton, twin gestation, and at the extremes of fetal growth 15. Measurement of TCD can be performed on most fetuses, irrespective of the fetal head shape 16.
Therefore, the aim of the present study is to assess the accuracy of TCD measurement in estimation of the GA during the second and the third trimester compared with the current fetal biometric measurements (FL and BPD).
We found that TCD was reliable in the prediction of GA, with concordance between the actual and the predicted GA by TCD throughout gestation with an acceptable degree of accuracy.
Other studies have reported the reliable use of TCD with other multiple biometric parameters in an attempt to find a method to determine the GA and identify the GA of fetus 17.
In a study carried out by Reece et al. 9, multiple biometric parameters were obtained, including the TCD using the electronic calipers of the machine. The findings reported by these authors indicate that growth of the TCD is not affected by intrauterine growth retardation; thus, this sonographic measurement may serve as a reliable correlate of GA against which potential deviations of growth may be compared 9.
In the study by Lee et al. 18, GA prediction intervals were derived from 270 normal fetuses between 14 and 40 weeks’ gestation for BPD, head circumference, abdominal circumference, FL, and TCD. TCD satisfactorily predicted GA for six fetuses with asymmetric intrauterine growth retardation and was associated with the least amount of underestimation bias compared with other ultrasonographic parameters 18.
In the present study, we found minimal differences between the TCD measurement–GA relationship and nomograms in other studies. The predicted GA in the third trimester was different in comparison with other studies by 0–3 weeks up to 30 weeks and by 0.5–6 weeks from 32 weeks of gestation.
The study carried out by Chavez et al. 19 showed that between 14 and 27 weeks’ gestation, there were no clinically significant differences between their nomogram and the previously published studies in terms of the predicted GA. However, predicted GA in the third trimester was considerably different on using their nomogram by 1–2 weeks from 28 to 30 weeks and by 4–6 weeks after 32 weeks 19.
In the present study, regression analysis indicated a strong linear relationship between TCD and GA, which makes the present study highly significant, and proved that TCD may serve as a reliable indicator of GA and fetal growth.
Similar to our findings, fetal imaging studies have shown that TCD correlates closely with GA and that the increase in TCD is linear during the second 20–22 and third trimester 5,14,19. Also, it has been shown that this correlation is not affected significantly by sex, fetal growth restriction, multiple pregnancy, or fetal macrosomy 9,21,23,24.
The study by Davies et al. 14 aimed to confirm the relationship between GA and TCD to define the prediction of GA by TCD and assess the reliability of TCD measurements. TCD was measured in a total of 221 infants with known GA. TCD correlates closely with GA and predicts GA to ±2.33 weeks. Measurements of TCD have excellent reproducibility 14.
The study by Goel et al. 16 was carried out on 50 antenatal patients (20–40 years of age) between 14 and 40 weeks of pregnancy attending the clinic for routine ultrasound examination. Ultrasonographic measurement of TCD was performed to assess GA. The regression analysis indicated a significant relationship between TCD and GA, indicating that TCD is a good marker for the estimation of GA 16.
The TCD measurement appears to be an accurate predictor of GA, even in the third trimester of pregnancy. It correlates closely with GA and predicts GA up to ±2 week in 83.3% of gestations. The suggested nomogram for TCD in the current study may be particularly useful for accurate dating of pregnancies in the third trimester. It is recommended to use TCD as an important ultrasound biometric parameter in normal singleton for the prediction of GA.
Conflicts of interest
There are no conflicts of interest.
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