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This study aims to investigate the male contribution to fertility and explore the underlying biological mechanisms. Specifically, we aim to (1) identify male factors associated with successful pregnancy, (2) develop a fertility index incorporating modifiable factors for both males and females to predict pregnancy rate and (3) explore the relationship of male modifiable factors with semen parameters and molecular characteristics.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Methods and analysis<\/jats:title>\n                    <jats:p>We will conduct an unmatched case-control study involving 240 couples with impaired male fertility (cases) and 240 couples with normal male fertility (controls). Between July 2024 and June 2026, we will recruit 480 eligible couples from KK Women\u2019s and Children\u2019s Hospital, Singapore. Male and female participants will complete questionnaires on sociodemographics, general health and lifestyle factors, and their anthropometry and body fat composition will be measured. Blood and semen samples from the male participants will be collected for biochemical, molecular and semen analyses. Predictive male factors will be identified using the least absolute shrinkage and selection operator method, accounting for female factors. We will construct a logistic regression model incorporating both male and female factors to derive a fertility index, which will be evaluated using cross-validation on subsets of the study population. Multivariable linear regression will be used to explore relationships between male modifiable exposures and semen parameters.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Ethics and dissemination<\/jats:title>\n                    <jats:p>The study protocol has received approval from the Centralised Institutional Review Board of SingHealth (2024\/2120), Singapore. Participants will provide written informed consent. Study results will be disseminated through conferences and peer-reviewed scientific journals.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Trial registration number<\/jats:title>\n                    <jats:p>\n                      <jats:ext-link xmlns:xlink=\"http:\/\/www.w3.org\/1999\/xlink\" ext-link-type=\"clintrialgov\" xlink:href=\"NCT06293235\">NCT06293235<\/jats:ext-link>\n                      .\n                    <\/jats:p>\n                  <\/jats:sec>","DOI":"10.1136\/bmjopen-2024-088143","type":"journal-article","created":{"date-parts":[[2025,1,16]],"date-time":"2025-01-16T04:02:42Z","timestamp":1737000162000},"page":"e088143","update-policy":"https:\/\/doi.org\/10.1136\/crossmarkpolicy","source":"Crossref","is-referenced-by-count":3,"title":["Investigating male factors and their relationships with reproductive health outcomes: a case-control study protocol for Towards Optimal Fertility, Fathering, and Fatherhood studY (TOFFFY) in Singapore"],"prefix":"10.1136","volume":"15","clinical-trial-number":[{"clinical-trial-number":"nct06293235","registry":"10.18810\/clinical-trials-gov"}],"author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-3719-3005","authenticated-orcid":false,"given":"Chee Wai","family":"Ku","sequence":"first","affiliation":[{"name":"Department of Reproductive Medicine, KK Women's and Children's Hospital, Singapore"},{"name":"Duke-NUS Medical School, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jun Wei","family":"Pek","sequence":"additional","affiliation":[{"name":"Temasek Life Sciences Laboratory, Singapore"},{"name":"Department of Biological Sciences, National University of Singapore, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yin Bun","family":"Cheung","sequence":"additional","affiliation":[{"name":"Program in Health Services and Systems Research and Centre for Quantitative Medicine, Duke-NUS Medical School, Singapore"},{"name":"Tampere Centre for Child, Adolescent and Maternal Health Research, University of Tampere and Tampere University Hospital, Tampere, Finland"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Melissa D\/O","family":"Tharmalingam Durgahshree","sequence":"additional","affiliation":[{"name":"Department of Reproductive Medicine, KK Women's and Children's Hospital, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Melinda","family":"Chan","sequence":"additional","affiliation":[{"name":"Department of Reproductive Medicine, KK Women's and Children's Hospital, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Yie Hou","family":"Lee","sequence":"additional","affiliation":[{"name":"Duke-NUS Medical School, Singapore"},{"name":"Singapore-MIT Alliance for Research and Technology, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Keith","family":"Godfrey","sequence":"additional","affiliation":[{"name":"MRC Lifecourse Epidemiology Unit, University of Southampton and Southampton University Hospitals NHS Trust, Southampton, UK"},{"name":"University of Southampton and University Hospital Southampton National Health Service Foundation Trust, National Institute for Health Research Southampton Biomedical Research Centre, Southampton, UK"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Fabian","family":"Yap","sequence":"additional","affiliation":[{"name":"Duke-NUS Medical School, Singapore"},{"name":"Department of Paediatrics, KK Women's and Children's Hospital, Singapore"},{"name":"Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jerry Kok Yen","family":"Chan","sequence":"additional","affiliation":[{"name":"Department of Reproductive Medicine, KK Women's and Children's Hospital, Singapore"},{"name":"Duke-NUS Medical School, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"See Ling","family":"Loy","sequence":"additional","affiliation":[{"name":"Department of Reproductive Medicine, KK Women's and Children's Hospital, Singapore"},{"name":"Duke-NUS Medical School, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"239","published-online":{"date-parts":[[2025,1,15]]},"reference":[{"key":"2025121512080362000_15.1.e088143.1","unstructured":"Department of Statistics Singapore . 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