Hi Dr. Wilson,

Based on my research, I would say that the best way is to make sure that you are in a true axial plane, at the level of BPD. So for us new students it is very important to keep the anatomical landmarks in mind and try to find different windows so that we can see the calvarium all across and are  not creating a false sign of pathology. We need to be mindful of shadowing and how different angles of ultrasound can change the appearance of structures. For lemon sign, the frontal indentation is bilateral and a lot of the times the baby will also have issues with the cerebellum where is it shaped like a banana and ventriculomegaly might be seen as well. There was an article I was reading that mentioned if baby is in the cephalic position then do coronal TV scan to check for cranial abnormalities (https://www.karger.com/Article/Fulltext/364806).Also, the lemon sign disappears in about the late second and third trimester, so if I had any doubt I would just document a possible lemon sign in my report and the patient can get rescanned just to make sure. I rather be safe than sorry. :)