What do the Proportion, Symmetry and Harmony scores actually mean?
Proportion measures how the width of the head compares to its length, Symmetry measures how closely the left side mirrors the right, and Harmony combines the two into one overall number — all three run 0 to 100, and all three come from the same single top-down photo. The one counter-intuitive part is Proportion: it peaks at a ratio in the middle of the typical range, so a very round head and a very long head can both score below a head that sits closer to the middle.
What does the Proportion Score tell me?
Proportion reads the ratio of the head's width to its length off the traced outline. It isn't a scale that simply rewards "rounder" or simply rewards "longer" — full marks sit at a target ratio in the middle of the typical range, and the score comes down by the same amount whichever direction the head departs from it. See the exact formula on the methodology page.
Why do a very round head and a very long head both score lower?
Because Proportion measures closeness to that target ratio, not "roundness" or "length" as their own separate scale. Picture the score as a hill with a single peak sitting at the target ratio: move away from the peak in either direction and the score falls off at the same rate. A head at one extreme and a head at the opposite extreme can land on the same Proportion Score even though they look nothing alike — what they share is being an equal distance from the middle, just in opposite directions. See why that middle ratio is the target for the reasoning behind it.
What does the Symmetry Score tell me?
Symmetry compares two diagonal measurements taken across the outline, one to the left of vertical and one to the right, and scores how close they are to each other. Every real head has some small, ordinary difference between its two sides, so a small tolerance band is built in before any points come off — the first 0.5% of difference costs nothing, and only the amount beyond that band affects the score. See the exact formula for the numbers behind it.
What does the Harmony Score tell me?
Harmony is the single number that combines Proportion and Symmetry — but not as a plain average. It's their geometric mean, so a weak score on one side pulls the combined number down more than an average would, and a strong score on the other side can't fully make up for it. See the formula and a worked example on the methodology page.
Is a higher score always better?
Within what these scores actually measure — proportion and symmetry read off a single photo — yes, higher generally means closer to the target ratio or closer to an even left-right match. That's also the limit of what they say: they're a measurement and a simulation, not a judgment about your baby's health. See what this tool can't tell you for where that line sits.
Curious how these numbers move once you start scanning regularly? See how to track changes month to month for the full approach.
For informational and entertainment purposes only. Consult a professional if you have health concerns.