The enigmatic bone reduction pattern of thalidomide embryopathy is set by sequential exit from the sensitive window

Thalidomide reduces the limb bones in a puzzling order, starting with the thumb, the radius and the humerus and then returning to the ulna and the ulnar digits. To understand that order we analysed 435 thalidomide-affected limbs from the published record (1962-2025), of which 269 were upper limbs - 142 human and 127 non-human primate. Reconstructed by forward-chain assembly the reduction order runs: Digit 1 [->] Radius [->] Humerus [->] Digit 2 [->] Ulna [->] Digit 3 [->] Digit 4 [->] Digit 5. The co-reductions resolved into two rules. Within a segment (autopod, zeugopod), no bone is reduced before every bone anterior to it. Among the most posterior bones of the three segments (digit 5, ulna, humerus): no bone is reduced before every bone proximal to it. Behind these antero-posterior (A-P) and proximo-distal (P-D) reduction rules stands a maturation wave advancing postero-anteriorly and disto-proximally. In developmental terms the bone precursors exit their sensitive windows after the autopod is built, in a biaxial order: from digit 5 towards digit 1, from the ulna to radius, and across segments from the autopod towards the stylopod (Digit 5 [->] Digit 4 [->] Digit 3 [->] Ulna [->] Digit 2 [->] Humerus [->] Radius [->] Digit 1). A co-reduction is therefore a snapshot of the prospective bones still sharing a sensitive window before their sequential biaxial maturation: exposure during elongation truncates the limb proximal to the progress zone, while later exposure reduces only the bones still immature at the time of exposure.