Re: Non‐visualization of the cavum septi pellucidi is not synonymous with agenesis of the corpus callosum. G. Malinger, D. Lev, M. Oren and T. Lerman‐Sagie. <i>Ultrasound Obstet Gynecol</i> 2012; 40: 165–170.

The cavum septi pellucidi (CSP) et Vergae is the hypoechoic fluid-containing rectangular space between the as yet unfused laminae of the septum pellucidum on standard axial views of the head until septal fusion occurs, typically in the third trimester, forming the single septum pellucidum. The utility and importance of CSP identification at prenatal ultrasound as a marker for normal development of midline structures including the corpus callosum has been affirmed by several investigators1. Failure to demonstrate the normal CSP at the mid-trimester anatomy scan should trigger detailed assessment of the brain and other anatomy and consideration of counseling and further investigations. What remains as a clinical problem is the significance of alterations in the appearance of the CSP, including unexpectedly large or small size and alteration in the appearance of its contents. In this issue of the Journal, Malinger and colleagues have reviewed the unusual occurrence of failure to see fluid in the CSP, in fetuses evaluated from 22 to 35 gestational weeks. It is reassuring that in most cases the finding was isolated and followed by normal outcome. Nonetheless, a few cases had problems, reaffirming the approach that any deviation from normal appearance should lead to further assessment. The embryology of the corpus callosum (CC) and CSP is complex2, 3. An issue that Malinger et al. raise is the range of normal sonographic appearances of the septa, CSP and fornices around the time of their initial formation in early pregnancy. Embryological reviews suggest that even up to 17–20 weeks the septal walls may be quite thick and leave little space for fluid2, 4. In contrast, at ultrasound, the CSP is described as a clear box with thin walls (septa) that become visible in some fetuses as early as 15 weeks and in all normal fetuses by 16–18 weeks5. Clarification of the normal early developmental sonographic appearance of this region is especially relevant today, with the increased emphasis on early anatomical assessment using very high-resolution transvaginal probes and multiple views (coronal and sagittal). Until clarification is achieved, caution should be exercised regarding counseling related to suspected CSP/CC variations prior to 18 gestational weeks. It is important to remember that as new and ever more sophisticated transducers and image processing are introduced, ‘new’ unexpected findings will be observed. These should be reported and managed with caution until their significance is established. A. Toi*, * University of Toronto, Princess Margaret Hospital, 610 University Avenue, Room 4-833, Toronto, Canada, M5G 2M9

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