Could excessive infant mattress firmness contribute to positional plagiocephaly in Japan? An international comparison and hypothesis paper

Background International infant sleep-surface standards primarily address the prevention of overly soft sleep surfaces using pass/fail test methods rather than specifying a directly comparable consumer firmness value in Newtons. No international standard establishes an upper firmness limit for infant mattresses. Japan has no infant-specific national mattress firmness standard. Objective To document the firmness (Newton value, N) of Japanese commercially available infant mattresses, compare these values against international regulatory frameworks and selected manufacturer recommendations, and propose a biomechanical hypothesis suggesting that excessive mattress firmness may contribute to the apparently higher prevalence of positional plagiocephaly (PP) reported in Japanese cohorts. Methods Firmness values (N; JIS K6400-2 Method A, 40% compression) were collected from publicly available manufacturer or retailer specifications for ten Japanese infant mattress products satisfying pre-defined inclusion criteria (the primary analysis dataset). Two additional products, identified outside the original search during revision, are reported as a sensitivity check only. International reference values were obtained from regulatory documents, selected European manufacturer guidance, and product documentation. PP prevalence data were extracted from peer-reviewed literature; cross-national comparisons are presented as illustrative only. Results Japanese infant mattress firmness ranged from 160 to 285 N (median 183 N, ten-product primary analysis). No direct numerical comparison with European density-based manufacturer recommendations is made, as no validated conversion between density (kg/m 3 ) and ILD firmness (N) exists. Five products had firmness at or below 176 N; four products exceeded 190 N. Of the ten products, six fell within the Korean reference-test-mattress specification range (170 ± 20 N = 150–190 N), indicating that the Japanese market does not consist uniformly of extreme outliers. This preliminary survey is restricted to ten products identified through publicly accessible manufacturer or retailer specifications; independent laboratory verification was not performed. Safety statement Existing evidence strongly supports firm sleep surfaces as a key measure for reducing sudden infant death syndrome (SIDS) risk. The present hypothesis concerns only the upper end of the firmness spectrum—specifically, whether surfaces that are excessively firm may carry biomechanical consequences distinct from those of appropriately firm surfaces. This paper does not recommend using softer mattresses, adding comfort toppers, or using positioning pillows or positioners. Hypothesis Excessive mattress firmness may reduce occipital contact surface area (increasing interfacial pressure: P = F / A ) and may reduce small spontaneous head-position changes during sleep (a direction that remains to be tested), thereby increasing sustained focal pressure on the immature, malleable calvaria and potentially contributing to the apparently higher PP prevalence reported in Japan. Conclusions Japanese infant mattresses surveyed had a median firmness of 183 N. Among the regulatory frameworks reviewed, Japan lacks an infant-specific national mattress firmness standard, and no international standard establishes an upper firmness limit. Independent market-wide testing and prospective outcome studies are warranted to determine whether an infant-specific upper firmness range should be considered.