http://fertility.ca/my-diagnosis/list-of-diagnoses/arcuate-subseptate-uterus/ - "A minor dip ... is called an arcuate cavity. As it becomes longer, we call it subseptate. If the dip is over 1 cm in size, it’s sometimes called a “true septum” – like a wall dividing the two halves of your uterus. Although bigger dips are more serious, there’s actually no medical consensus yet on where the line between “normal” and “abnormal” should be drawn. In fact, the decision to label your dip “arcuate”, “subseptate” or “septum” isn’t standardized."

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