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This impression, however, may be an artefact of past adoption of anthropocentric measurements that underestimate birth challenges in non-human primates. Here we re-evaluate cephalopelvic proportions using species-specific three-dimensional data of the pelvic inlet along with neonatal cranial dimensions from a broad sample of extant primates. Results reveal that maternal body size is a key factor to consider. A tight cephalopelvic fit occurs in species with proportionately larger neonates, smaller pelves or a combination of both. The latter is the case in humans, producing the tightest fit among extant apes, but a similar combination of factors explains much more extreme cephalopelvic proportions in other species. Our findings reveal a diversity of obstetrical dilemmas across primates.<\/jats:p>","DOI":"10.1038\/s41559-026-03102-5","type":"journal-article","created":{"date-parts":[[2026,6,29]],"date-time":"2026-06-29T15:02:17Z","timestamp":1782745337000},"page":"1603-1613","update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Comparative primate analysis shows that humans are not unique in having a tight cephalopelvic fit at birth"],"prefix":"10.1038","volume":"10","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-2504-3886","authenticated-orcid":false,"given":"Nicole","family":"Torres-Tamayo","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Stefan","family":"Schlager","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7849-1936","authenticated-orcid":false,"given":"Eishi","family":"Hirasaki","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6883-8964","authenticated-orcid":false,"given":"Tim D.","family":"Smith","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Todd C.","family":"Rae","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2895-9718","authenticated-orcid":false,"given":"Lia","family":"Betti","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2026,6,29]]},"reference":[{"key":"3102_CR1","doi-asserted-by":"publisher","first-page":"401","DOI":"10.1002\/ajpa.1330070307","volume":"7","author":"AH Schultz","year":"1949","unstructured":"Schultz, A. 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