Gestational age (GA), according to the last menstrual period (LMP), is defined as the weeks beginning from the first day of LMP before conception. [1] The accurate knowledge of GA is a keystone in an obstetrician’s ability to successfully manage the antepartum care of a patient and is of critical importance in antenatal tests and the successful planning of appropriate therapy or intervention. Failure can result in iatrogenic prematurity, which is associated with increased prenatal morbidity and mortality. [2] [3]

Accurate determination of GA is an important and potentially life-saving step in providing optimal medical care to pregnant women in the emergency department. Critical management decisions, such as the use of cesarean section, can be made confidently only when fetal viability can be assessed reliably. Traditionally, estimations of GA are based on the patient’s recollection of her LMP; however, previous research suggests that this may be inaccurate. [4]

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. [5] In the third trimester, the reliability of any single ultrasound parameter is poor. [6]

Ultrasound (US) assessment for GA is becoming increasingly important. Many parameters can be used for establishing GA, for example, biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). Recently, the evaluation of the posterior fossa of the fetal cranium has been accepted as part of routine obstetrical estimations; previously, the BPD had been described as a reliable method of determining GA. [7] [8] While 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, including HC, AC, FL, foot length, ear size, orbital diameters, cerebellum diameter, and others. [9] - [12] The fetal cerebellum grows progressively along with gestational age; therefore, ultrasonography examination can predict gestational age at any trimester. Parameters such as BPD are thought to compute gestational age more correctly when performed at an earlier gestation. [13] The transverse cerebellar diameter (TCD) provides a dependable measure for estimating gestational age in pregnancies affected by intrauterine growth restriction, showing close agreement with early dating ultrasound. This underscores its utility as an alternative assessment tool when early dating scans are either unavailable or unreliable. [14] This study establishes transverse cerebellar diameter (TCD) as the most reliable biometric parameter for estimating gestational age (GA) in the second and third trimesters. Unlike other measures, TCD remains accurate even when the last menstrual period is unknown, no first-trimester scan is available, the assessment begins late, or fetal head shape is altered. [15] In late pregnancy, the fetal transcerebellar diameter (TCD) has been demonstrated to be a reliable biometric parameter, exhibiting a strong correlation with gestational age estimated from the last menstrual period. [16]

The study has been designed as a prospective cross-sectional (observational) study in Khartoum State, Sudan. The fetal cerebellum of 384 pregnant women was examined at various stages of pregnancy at Al Saudi Maternity Teaching Hospital, Al Rasheed Medical Center, and The National Ribat University Hospital. The study population consisted of 384 pregnant Sudanese women, ranging in gestational age from 5 to 40 weeks. And referred to an obstetric clinic. Aged between 15 and 45 years, all women who satisfied the inclusion criteria stated in this study had a regular menstrual cycle, certainty about the time of their last menstrual period, a normal pregnancy, and a viable singleton included in the study population. On the other hand, the study excluded those unsure of an accurate memory of the last menstrual period, fetal congenital abnormalities, failure to visualize/ measure the fetal cerebellum, pregnancy-induced hypertension, diabetes mellitus, cardiac diseases, thyroid disease, repeated cesarean section, intrauterine fetal death, chronic renal disease, and renal failure. The study population, representing different Sudanese tribes, was limited to the state of Khartoum. Trans cerebellar diameter was measured by identifying the cerebellum in the posterior fossa and measuring it from the outer to the outer extent. The thalamic plane used for the biparietal diameter was then obtained. The abdominal circumference was obtained from a transaxial view at the level of the junction of the umbilical vein and left portal vein. The main outcome measures noted were trans cerebellar diameter, other parameters noted were biparietal diameter, femur length, abdominal circumference, and head circumference.

Reliability statistics have been made by using Cronbach’s Alpha, for all (Fetal age per week depending on LNMP, TCD, and median longitudinal measurement), which was 0.879 and validity 0.938, for individual parameters, GA reliability 0.846 and validity 0.920, TCD reliability 0.802 and validity 0.896, and median longitudinal measurement reliability 0.834 and validity 0.913.

The mean fetal age, depending on LNMP per week, was 26.48 ± 6.824, and ranged between 13 and 41, as shown in Table 1 . As well as the mean of TCD was (28.047 ± 7.6244), and ranged between (7.0 - 56.6) mm. Also, the mean of MLM was (16.870 ± 5.1149) and ranged between (2.6 - 33.0) mm. The mean FL was (63.369 ± 8.4450) and ranged between (18.0 - 82.0) mm. At the same time, the mean fetal BPD was (68.369 ± 17.3129) and ranged between 21.0 mm and 95.9 mm. The mean fetal head HC was 191.036 (±37.8012) and ranged between 91.0 mm and 262.0 mm. The mean fetal AC was 283.992 (±39.9485) and ranged between 116.0 mm and 364.2 mm, as shown in Table 1 .

This study showed that the predictor TCD has a linear correlation with FA depending on LMP (R = 0.720), R Square = 0.518. Adjusted R Square = 0.517 and Standard error of the Estimate = 4.744, respectively. These results were obtained from ANOVA and correlation coefficients, as shown in Table 2 , Table 3 , and Figure 3 .

The GA, depending on the LMP period, could be obtained from the following equation by using TCD:

Y = 8.414 + 0.644 × X

where Y = fetal age, X = transcerebellar diameter.

The study showed that the predictor (TCD) has a linear correlation with FA, which, depending on LMP, R = 0.429, R Square = 0.184, Adjusted R Square = 0.180, and Standard error of the Estimate = 7.6463, respectively. These results were obtained from ANOVA and correlation coefficients, as shown in Table 4 , Table 5 , and Figure 4 .

The fetal ages, depending on the LMP, could be obtained from the following equation by using TCD:

Y = 42.198 + 0.659 × X

where Y = fetal age, X = transcerebellar diameter.

The study showed that the predictor TCD has a linear correlation with FA depending on LMP and fetal BPD: R = 0.819, R Square = 0.671, Adjusted R Square = 0.670, and Standard error of the Estimate = 9.9475, respectively. These results were obtained from ANOVA and correlation coefficients, as shown in Table 6 , Table 7 , and Figure 5 .

The FA, depending on the LMP, could be obtained from the following equation:

Y = 15.959 + 1.865 × X

where Y = fetal age, X = TCD.

The study showed that the predictor (TCD) has a linear correlation with FA depending on LMP and fetal HC: R = 0.763, R Square = 0.583, Adjusted R Square = 0.580, and Standard error of the Estimate = 24.4889. These results were obtained from ANOVA and correlation coefficients, as shown in Table 8 , Table 9 , and Figure 6 .

The FA, depending on the LMP with HC, could be obtained from the following equation formula:

Y = 55.744 + 6.289 × X

where Y = fetal age, X = TCD.

The FA, depending on the LMP, could be obtained from the following equation:

Y = 15.959 + 1.865 × X

where Y = fetal age, X = TCD.

The study showed that the predictor (TCD) has a linear correlation with fetal age depending on LMP and fetal AC: R = 0.336, R Square = 0.113, Adjusted R Square = 0.109, and Standard error of the Estimate = 37.7107, respectively. These results were obtained from ANOVA and correlation coefficients, as shown in Table 10 , Table 11 , and Figure 7 .

The FA, depending on the LMP with AC, could be obtained from the following equation:

Y = 201.154 + 2.506 × X

where Y = fetal age, X = TCD.

The study showed that the predictor (TCD) has a linear correlation with FA depending on LMP and MLM: R = 0.792, R Square = 0.628, Adjusted R Square = 0.627, and Standard error of the Estimate = 3.1238. These results were obtained from ANOVA and correlation coefficients, as shown in Table 12 , Table 13 , and Figure 8 .

The FA, depending on the LMP, could be obtained from the following equation by using TCD:

Y = 1.960 + 0.532 × X

where Y = fetal age, X = transcerebellar diameter.

The present findings showed that the mean ± SD of the GA was (26.5 ± 6.8) weeks, similar to measurements obtained by Mahmoud et al., [17] who reported in their study that the mean ± SD of the GA was (26.8 ± 6.8) weeks. The present study showed the mean ± SD of the TCD was 28.05 ± 7.62; this disagrees with the result obtained by [18] , who reported in their study that the mean TCD was 16.7 mm at 14 - 20 weeks, 23.2 mm at 21 - 27 weeks, and 30.6 mm at 28 - 34 weeks. In spite of the strong correlation between the TCD and GA as compared to the previous studies, the correlation coefficient (r = 0.720) in the present study was less as compared to [17] (r = 0.94), [19] (r = 0.972), [20] (r = 0.971), [18] (r = 0.946), [21] (r = 0.991), and [22] (r = 098). Several reasons could explain these differences. These include the type of study (longitudinal vs. cross-sectional), quality of the US machine (new vs. old), and characteristics of subjects (only uncomplicated pregnancies vs. all pregnancies).

Furthermore, there was a strong positive correlation in this study between the TCD and the other parameters, such as BPD (r = 0.819 and P = 0.000), MLM (r = 0.792 and P = 0.000), and HC (r = 0.763 and P = 0.000). However, the correlation was not that strong between TCD and FL (r = 0.429 and P =0.000) and AC (r = 0.336 and P = 0.000). The finding of the present study was lower than that of [20] , who studied 301 pregnant women, and found the correlation of TCD with BPD was (r = 0.960), with HC (r = 0.979), with AC (r = 0.980), and with FL (r = 0.976).

Regarding the MLM of the cerebellum, [23] studied 10605 fetuses with GA ranging between 20 - 35 weeks, and they reported that the mean ± SD of the cerebellar vermis was 12.7 ± 1.6. Furthermore, the vermis longitudinal diameter showed a linear correlation with GA (r = 0.87). Our study observed less than the measurements obtained by [24] , who studied 293 healthy fetuses between 19 and 34 weeks. They also reported a linear correlation with GA (r = 0.95; P = 0.001). In addition, [25] studied 328 normal singleton pregnancies between 18 and 33 weeks of gestation (r = 0.887, p = 0.0001). [26] A retrospective study examined 516 fetuses with a normal posterior cranial fossa, between 21 and 35 weeks of gestation, by using three different imaging modalities to measure vermis biometry: 2D ultrasound, 3D ultrasound, and MRI. The number of fetuses in the study group was 193 for 2D ultrasound, the mean was 19.28 mm (r = 0.839), 172 for 3D ultrasound, with the mean was 17.80 mm (r = 0.806), and 151 for MR imaging, the mean was 21.34 mm (r = 0.565), and (P = 0.001) for the three groups. Furthermore, measurements of the cerebellar vermis correlated linearly with BPD, HC, and TCD (r = 0.82, P = 0.0001) as reported by [27] .