Fetal central nervous system (CNS) abnormalities are relatively common (11). Although some patients are at high risk for fetal CNS abnormalities due to family history or exposure to teratogens, such as congenital infections, the vast majority of fetal CNS abnormalities occur in patients without any fetal or familial/maternal risk (12). Therefore, evaluation of the fetal CNS during routine mid-trimester USG screening plays an important role in the prenatal diagnosis of these abnormalities and is the gold standard for their detection (11).
CC malformations may cause neurodevelopmental impairment in different dimensions (13). With the widespread use of high-resolution USG devices, early diagnosis of anomalies provides an advantage. In this study, we aimed to present the reference values of CC length between 16 and 25 weeks of gestational age, which show a positive correlation with advancing gestational age.
One of the important advantages of antenatal USG is the early detection of anomalies. In a recent study, Weisbach et al. established reference intervals for CC length measurements at 13–19 early gestational weeks and defined a case as a CC anomaly at 14 weeks. In the same study, they were only able to visualize CC in the appropriate section in 80% of cases (14). To our knowledge, this is the only study that performed early screening at 13–15 weeks of gestation. Apart from this study, there are only two studies in the literature that established CC reference intervals at 16 and 17 weeks of gestation (15; 16). Although there are many CC nomogram studies in the literature, there are only three studies that investigated reference intervals at early gestational weeks, such as 16–17 weeks of gestation. We believe that our study can be an important reference in this regard when there are not enough studies in the early gestational weeks.
In a comparison of 2D and 3D USG techniques for CC imaging in the literature, it was argued that 3D CC imaging was easier to obtain than 2D imaging. However, it was found that 3D imaging was not sufficient to show the limitations of 2D imaging (17). Harreld et al. compared the results obtained using MRI and USG for CC length measurement. As a result of the study, they argued that the measurements of both imaging techniques were equivalent (18). In our study, the measurements were performed in all cases using the 2D USG technique by providing a suitable CC image.
Whenever possible, patients suspected of having a congenital anomaly should be referred to advanced centers for evaluation by competent personnel and definitive diagnosis (19). This evaluation includes a detailed examination of the CNS, commonly referred to as "fetal neurosonography" (20). However, to our knowledge, there is no standardization in the reporting of suspicious findings during routine scans. We believe that clinicians should have a good knowledge of the norm for appropriate interpretation and a standardized description of abnormal USG findings. Ultimately, a standardized approach may help reduce the number of false-positive cases. Therefore, we believe that this study will guide clinicians.
In reviewing the literature, it is clear that CC measurement is of great importance. However, there are no studies to indicate which technique and method is superior in CC measurement. In our study, CC length was measured in all fetuses using a 2D convex USG probe. Measurement was achieved in 100% of cases. We suggest that CC length can be easily measured in healthy fetuses with normal anatomy using a 2D convex probe in the sagittal plane. We also believe that the earliest gestational week for anatomical assessment of the CC is 16 weeks. Evaluation of the CC at earlier weeks may lead to misinterpretation. As a result, unnecessary invasive procedures may be performed. This may lead to complications such as unwanted pregnancy loss.
Although we argue that our study has strengths in terms of contributing to the literature, it also has some limitations. The strengths of this study include the prospective design of the study, the fact that the person performing the measurement was an expert in his/her field, the high quality of the USG device used, the fact that all fetuses were healthy, the gestational weeks were selected as appropriate weeks for CC length imaging and all parents were of Turkish origin. The limitations of our study are that it was single-centered, fetuses with abnormal examination findings and multiple pregnancies were not included in the study, no comments were made on CC morphology, and we did not have information about postnatal cognitive intelligence.