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RO The boundary is drawn against the worst case, not the current one. The safe donor area is mapped as though the person will one day reach the most advanced pattern their history allows. A man at Norwood 3 today is given the same boundary as a man at Norwood 6, because the hair above that line has not been spared — it has simply not been reached yet.
It is a probability, not a landmark. No suture line, muscle or bone marks the edge. It is inferred from the present pattern, the rate of change, family history, age, and the miniaturised hairs visible under magnification. Two donor areas that look identical can have different safe zones, because the trajectories behind them differ.
| Region | How it behaves over a lifetime |
|---|---|
| Central occipital | The most stable band; the core of every donor plan |
| Lateral parietal, above the ears | Stable in most people, but narrows as the pattern advances |
| Upper edge, toward the crown | Often counted as donor, yet thins together with the crown |
| Temporal | Fine and sparse, prone to recession; rarely a true donor |
The boundary moves in one direction only. As loss progresses the safe zone contracts. Grafts taken from its upper edge at twenty-five can be standing in bald scalp at forty-five, thinning along with everything around them — on a head whose donor has already been spent. Planning against the eventual pattern rather than the present one is what prevents that.
Related terms
- Donor area anatomy
- Donor overharvesting
- Donor area depletion
- Graft vs hair count
- Retrograde alopecia
- Diffuse unpatterned alopecia
For how the donor area is assessed and protected, see the donor area guide. For the pattern scale the boundary is judged against, see the Norwood scale.
