In the case of CSP, its dimensions were also measured in weeks 20 to 30 of gestation. These ranges (in terms of gestation weeks) can be used for fetal developmental monitoring. The important thing is that CSP should follow a regular growth pattern at the specified gestational age. The lack of growth of the dimensions of the CSP as observed during this time period or even its irregular or interrupted growth should raise doubts about the spectrum of underlying anomalies. Increasing the width of the CSP relative to its length is also important and only values less than 1.5 can be considered benign if other components of the anterior complex, especially the corpus callosum, were normal. Serial ultrasound is useful in measuring the width-to-length ratio of CSP and examining its decreasing course in such embryos. Another point about CSP was that in all of our studies the area inside the CSP was completely echolucent, so any echogenicity within it should be reported. Any significant increase, or disproportionate size of the ventricles, should be detected. Considering the individual components of the anterior complex and ensuring the natural process of their formation can be helpful. One of the unknown and questioned views in this study is the rectangular view of the anterior horns of the lateral ventricles. Of course, none of the normal embryos in our study had this view. And, as noted in one study, a more accurate evaluation is suggested if this variation is observed. However, such findings should not be interpreted as definitive pathology in the first place, and one suggestion for these conditions is to conduct more extensive studies to find possible natural variations. In our study, in all cases, the direction of the anterior HIF position in the midline was perpendicular to the callosal sulcus, so the deviation of these structures, although very minor, appears to be strongly abnormal and should be the starting point for further investigation. In the study of developing embryos, one of the major targets under consideration is the anterior brain complex, a group of anatomical structures that are clearly visible in the routine imaging plan at the level(?) of lateral ventricular of the brain(12). This evaluation is important for finding midline brain anomalies. In the present study, we paid particular attention to the properties, appearance, and mode of relationship of each component of the anterior complex in order to express a pattern of the natural range of these components in the timeframe considered in pregnancy. To study each part of the anterior complex, in the Axial Plan, we used sonographic findings. The present study is unique in that, unlike previous articles, reference values and quantitative ranges have been provided weekly for CSP. During ultrasound surveys in the 20 to 30 weeks of gestation, we provided a numerical and average definition range for this important diagnostic landmark. In a study by Vinal and colleagues in 2015, they concluded that the structures of the anterior brain complex are partially visible around week 20 of gestation(9). Our study also showed this gradual development. Also, with respect to the regular increase in the longitudinal and transverse dimensions of the CSP, we conclude that the growth of this anatomical structure along with pregnancy progression should be regular and consistent. Therefore, the absence of an increasing trend of CSP dimensions in consecutive second- trimester surveys, even if other brain biomarkers and landmarks are normal, requires more thorough scanning and more detailed investigations. In Karl's study, in many fetuses with partial agenesis of the corpus callosum (PACC), the ratio of CSP to rectangular forms was less than 1.5(4). In our study for the CSP Ratio index, the mean and standard deviation were 1.98 ± 038, and in most of the fetuses we examined this value was not less than 1.5. In only three fetuses this value was lower than 1.5: 1.33, 1.34, and 1.46 that were at weeks 21, 22, and 22 of pregnancy, respectively. These embryos were normal and during the follow-up ultrasound scan the index increased to above 1.5 and showed no pathologic change or deviation of CSP relative to midline. Therefore, the calculation of CSP Ratio for second trimester embryos, along with the observation of CSP shape and ultrasound appearance and recording of its dimensions (especially to increase the sensitivity of detection of spectrum of callosal agenesis), seems reasonable. Our conclusion is that in order to be able to make a difference in the CSP view, we need to verify other parameters such as ventricular atrium diameter, extension of the anterior IHF, callosal solcus and etc. and confirm that they are normal. The absence of CSP after 20 weeks of pregnancy should not be considered a technical issue, and instead of postponing it to subsequent weeks, we should look for underlying etiology and associated pathologies. A major concern is the observation of the interhemispheric fissure deviation associated with the absence of cavum septum pellucidum, which should give rise to a strong suspicion of a wide range of midline brain malformations. A study by Vinurel suggested that attention to the structure of the anterior IHF and its extension in routine pregnancy scans would enhance the diagnostic sensitivity for finding brain anomalies (especially midline anomalies)(21). In our study, it was found that all embryos we report normal along with observation and assurance of the formation of the CSP structure should also have anterior interhemispheric fissure exactly in the midline. Also in the study by Vinurel et al. it was found that IHF deviation occurs under different pathological conditions(21). While in some cases IHF deviation is clearly a warning sign for underlying brain anomaly, it is unfortunately in some cases associated with findings that are easily overlooked. It was stated in previous studies that IHF was described as "distorted" if the antero-posterior lineage(?) did not exhibit, and was described as "widening" if distanced between hemispheres(9). In our study, all cases of callosal sulcus were perpendicular to its anterior IHF; therefore, in the absence of the T-shape characteristic of CS and anterior IHF, clues should be sought for midline malformations. In the same study, in some patients with developmental disorder, the anterior horns had a "rectangular" lateral ventricle. Therefore, it was avoided to describe this view as a normal variation for the anterior horns of the lateral ventricles and to suggest a triangular or comma-shaped view as normal morphology. In our study, in none of the cases, the anterior horn was seen in the lateral ventricle of the rectangular view, and all were comma-shaped and symmetrical. Another variable that we measured in the embryos of the 20th to 30th weeks of gestation was lateral ventricular atrium values (in the ventricular trigone area), which showed that there was no significant increase in ventricular atrium size as pregnancy progressed. Therefore, in dramatic increase of lateral ventricular diameters, it is better to examine the evolving structures of the midline (especially the corpus callosum) in greater detail. Concerning the geometrical shape of the CSP, in previous studies, 73% of the embryos had a square shape view and the remaining 27% had a triangular shape view(9). In our study there were 97 fetuses (91.5%) with rectangular CSP and 9 (8.5%) cases with triangular CSP. However, it is interesting that in Sartori et al.'s study about anatomic variations of septum pellucidum, in the definition of cavum septum pellucidum, only the triangular view for CSP was described(22).