title: Who has sex with whom? Characteristics of heterosexual partnerships reported in a national probability survey and implications for STI risk.
abstract: BACKGROUND
  Sexually transmitted infection (STI) risk is determined both by partner numbers and partnership characteristics. Studies describing only recent partnership(s) overestimate long-term partnerships and underestimate the contribution of casual partnerships to STI transmission in populations. We describe all heterosexual partnerships in the past year in terms of partnership type, age and geographical mixing and how these characteristics relate to condom use.
  
  
  METHODS
  Probability sample survey of 11 161 men and women aged 16-44 resident in Britain, 1999-2001. Computer-assisted self-interviews asked respondents about partner numbers and detailed questions about their three most recent partnerships. We weight these data to represent partnerships for which detailed questions were not asked to present estimates for the population of partnerships.
  
  
  RESULTS
  Of 15 488 heterosexuals partnerships, 39.1% (95% CI 36.6-41.7%) of men's partnerships were 'not (yet) regular' vs 20.0% (95% CI 18.2-21.9%) of women's partnerships. While condoms were used at last sex in 37.1% (95% CI 35.0-39.3%) of men's and 28.8% (95% CI 27.1-30.6%) of women's partnerships, and for 55.3% (95% CI 52.6-58.0%) of first sex with new partners, these proportions declined with age. When partnerships involved an age difference of 5+ years [26.2% (95% CI 23.0-29.6%) of men's and 36.5% (95% CI 33.0-40.1%) of women's partnerships], condoms were less commonly used at first sex than when partners were closer in age [44.1% (95% CI 39.1-48.4%) vs 60.8% (95% CI 57.3-64.2%)]. Sex occurred within 24 h in 23.4% (95% CI 19.7-27.5%) of men's and 10.7% (95% CI 8.3-13.6%) of women's partnerships.
  
  
  CONCLUSIONS
  A substantial minority of partnerships in the population is casual. The proportion of partnerships not protected by condoms is high, especially for partnerships involving larger age differences and people in their 30s and 40s. Condom use with new partners needs to be promoted among all age-groups.
authors.affiliation: Centre for Sexual Health & HIV Research, Research Department of Infection & Population Health, University College London, Mortimer Market Centre, London WC1E 6JB, UK. cmercer@gum.ucl.ac.uk
authors.forename: Catherine H
authors.initials: CH
authors.lastname: Mercer
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Copas
authors.forename: Pam
authors.initials: P
authors.lastname: Sonnenberg
authors.forename: Anne M
authors.initials: AM
authors.lastname: Johnson
authors.forename: Sally
authors.initials: S
authors.lastname: McManus
authors.forename: Bob
authors.initials: B
authors.lastname: Erens
authors.forename: Jackie A
authors.initials: JA
authors.lastname: Cassell
mesh_terms: D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000367:Age Factors; D017280:Condoms / Q000706:statistics & numerical data; D005260:Female; D006306:Health Surveys; D020010:Heterosexuality / Q000523:psychology / Q000706:statistics & numerical data*; D006801:Humans; D008297:Male; D012307:Risk Factors; D012309:Risk-Taking; D012737:Sex Factors; D012747:Sexual Partners; D012749:Sexually Transmitted Diseases / Q000635:transmission*; D006113:United Kingdom; D047568:Unsafe Sex / Q000706:statistics & numerical data; D055815:Young Adult