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The Human Phenotype Ontology (HPO) is downloaded weekly from The file downloaded is considered the "last stable build" available for the ontology. For more about the HPO, view their website at

Term:Lemon sign
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Accession:HP:0032269 term browser browse the term
Definition:The lemon sign refers to the shape of the fetal skull at ultrasonography (US) when the frontal bones lose their normal convex contour and appear flattened or inwardly scalloped. This gives the skull a shape that is said to resemble a lemon.. The sign is seen on transverse sonograms of the fetal cranium obtained at the level of the ventricles.
Comment:This sign represents the loss of the normal convex contour of the frontal bones, with flattening or inward scalloping, seen on a transverse fetal sonogram obtained at the biparietal diameter level. It has a strong association with spina bifida and is very useful for detecting this condition before 24 weeks of gestation in high-risk patients. It has high sensitivity and specificity. The lemon sign may disappear as gestational age advances and hence it is less reliable after 24 weeks. The sign is not specific for spina bifida and has also been seen in encephalocele, Dandy-Walker malformation with encephalocele, thanatophoric dysplasia, cystic hygroma, diaphragmatic hernia, corpus callosal agenesis, hydronephrosis, and umbilical vein varix.

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  Human phenotype 0
    Phenotypic abnormality 0
      Abnormality of prenatal development or birth 0
        Fetal anomaly 0
          Abnormal fetal morphology 0
            Abnormal fetal skeletal morphology 0
              Lemon sign 0
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