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Rates of early-onset and late-onset HELLP syndrome (&lt;34\u2009vs \u226534 weeks, respectively) were calculated per 1000 ongoing pregnancies at 20 and 34 weeks\u2019 gestation, respectively. Cox regression was used to assess the associations between risk factors (eg, BMI, maternal age and parity) and early-onset versus late-onset HELLP syndrome.<\/jats:p><\/jats:sec><jats:sec><jats:title>Main outcome measures<\/jats:title><jats:p>Early-onset and late-onset HELLP syndrome.<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>The rates of HELLP syndrome per 1000\u2009women were 2.8 overall (1116 cases among 391\u2009941\u2009women), and 1.9, 2.5, 3.2 and 4.0 in underweight, normal BMI, overweight and obese categories, respectively. Overall, gestational age-specific rates of HELLP syndrome increased with prepregnancy BMI. Obesity (compared with normal BMI) was more strongly associated with early-onset HELLP syndrome (adjusted HR (AHR) 2.24 (95% CI 1.65 to 3.04) than with late-onset HELLP syndrome (AHR 1.48, 95%\u2009CI 1.23 to 1.80) (p value for interaction 0.025). Chronic hypertension, multiple gestation, bleeding (&lt;20 weeks\u2019 gestation and antepartum) also showed differing AHRs between early-onset versus late-onset HELLP syndrome.<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusions<\/jats:title><jats:p>Prepregnancy BMI is positively associated with HELLP syndrome and the association is stronger with early-onset HELLP syndrome. Associations with early-onset and late-onset HELLP syndrome differed for some risk factors, suggesting possible differences in aetiological mechanisms.<\/jats:p><\/jats:sec>","DOI":"10.1136\/bmjopen-2023-079131","type":"journal-article","created":{"date-parts":[[2024,3,23]],"date-time":"2024-03-23T16:40:28Z","timestamp":1711212028000},"page":"e079131","update-policy":"https:\/\/doi.org\/10.1136\/crossmarkpolicy","source":"Crossref","is-referenced-by-count":6,"title":["Prepregnancy body mass index and other risk factors for early-onset and late-onset haemolysis, elevated liver enzymes and low platelets (HELLP) syndrome: a population-based retrospective cohort study in British Columbia, Canada"],"prefix":"10.1136","volume":"14","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-7939-7316","authenticated-orcid":false,"given":"Li Qing","family":"Wang","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7704-1677","authenticated-orcid":false,"given":"Jeffrey N","family":"Bone","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Giulia M","family":"Muraca","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Neda","family":"Razaz","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"K S","family":"Joseph","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1220-310X","authenticated-orcid":false,"given":"Sarka","family":"Lisonkova","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"239","published-online":{"date-parts":[[2024,3,23]]},"reference":[{"key":"2024043021564403000_14.3.e079131.1","doi-asserted-by":"crossref","DOI":"10.1136\/bmj.l2381","article-title":"Pre-eclampsia: pathophysiology and clinical implications","volume":"366","author":"Burton","year":"2019","journal-title":"BMJ"},{"key":"2024043021564403000_14.3.e079131.2","doi-asserted-by":"publisher","DOI":"10.1016\/j.bpobgyn.2011.01.006"},{"key":"2024043021564403000_14.3.e079131.3","doi-asserted-by":"publisher","DOI":"10.1016\/S2214-109X(14)70227-X"},{"key":"2024043021564403000_14.3.e079131.4","doi-asserted-by":"publisher","DOI":"10.1161\/HYPERTENSIONAHA.108.117358"},{"key":"2024043021564403000_14.3.e079131.5","doi-asserted-by":"crossref","first-page":"270","DOI":"10.5468\/ogs.2020.63.3.270","article-title":"A study to compare maternal and perinatal outcome in early vs. late onset preeclampsia","volume":"63","author":"Wadhwani","year":"2020","journal-title":"Obstet Gynecol Sci"},{"key":"2024043021564403000_14.3.e079131.6","doi-asserted-by":"publisher","DOI":"10.1016\/j.ajog.2013.08.019"},{"key":"2024043021564403000_14.3.e079131.7","doi-asserted-by":"publisher","DOI":"10.1016\/j.ejogrb.2012.09.026"},{"key":"2024043021564403000_14.3.e079131.8","doi-asserted-by":"publisher","DOI":"10.1016\/S0002-9378(99)70022-0"},{"key":"2024043021564403000_14.3.e079131.9","doi-asserted-by":"crossref","first-page":"709","DOI":"10.1111\/aogs.13322","article-title":"HELLP syndrome, risk factors in first and second pregnancy: a population-based cohort study","volume":"97","author":"Malmstr\u00f6m","year":"2018","journal-title":"Acta Obstet Gynecol Scand"},{"key":"2024043021564403000_14.3.e079131.10","doi-asserted-by":"publisher","DOI":"10.1111\/j.1523-1755.2005.00316.x"},{"key":"2024043021564403000_14.3.e079131.11","doi-asserted-by":"publisher","DOI":"10.1016\/j.ajog.2005.02.113"},{"key":"2024043021564403000_14.3.e079131.12","doi-asserted-by":"crossref","first-page":"81","DOI":"10.1042\/CS20060015","article-title":"BMI: new aspects of a classical risk factor for hypertensive disorders in pregnancy","volume":"111","author":"Leeners","year":"2006","journal-title":"Clin Sci"},{"key":"2024043021564403000_14.3.e079131.13","doi-asserted-by":"crossref","first-page":"133","DOI":"10.1097\/AOG.0000000000000591","article-title":"Risk of adverse pregnancy outcomes by prepregnancy body mass index: a population-based study to inform prepregnancy weight loss counseling","volume":"125","author":"Schummers","year":"2015","journal-title":"Obstet Gynecol"},{"key":"2024043021564403000_14.3.e079131.14","doi-asserted-by":"publisher","DOI":"10.1097\/01.ede.0000254119.99660.e7"},{"key":"2024043021564403000_14.3.e079131.15","doi-asserted-by":"crossref","DOI":"10.1371\/journal.pone.0223888","article-title":"Increased BMI has a linear association with late-onset preeclampsia: a population-based study","volume":"14","author":"Robillard","year":"2019","journal-title":"PLoS One"},{"key":"2024043021564403000_14.3.e079131.16","unstructured":"World Health Organization . 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