A high proportion of people are not using condoms when they have sex with a new partner, according to a new study of heterosexual partnerships among British men and women. Among people in their 30s and 40s, and in partnerships where there is an age difference of five or more years, condom use is particularly low.
In view of the rising rates of sexually transmitted infections (STIs), the authors of the study say that condom use needs to be promoted to all age groups, and not just to young people.
The research, published online in the International Journal of Epidemiology today (Wednesday 12 November), looks at all heterosexual relationships experienced in the previous 12 months by 11,161 men and women interviewed for the second British National Survey of Sexual Attitudes and Lifestyles (Natsal 2). 
Dr Catherine Mercer, a lecturer in the Centre for Sexual Health & HIV Research, University College London (UK), said: "To the best of our knowledge this is the first research to take account of all heterosexual partnerships and not just people's most recent partnerships, which tend to be more established partnerships such as marriages and cohabitations. Our study ensures accurate representation of casual partnerships, which are known to be important in the transmission of STIs. People with large numbers of partners contribute disproportionately to STI transmission in populations."
Dr Mercer and her colleagues analysed data from interviews with 11,161 people, of whom 6,399 were women, carried out between May 1999 and February 2001. Interviewees were asked about their three most recent partnerships, and the analysis focused on partnerships in the past year. Questions included ones on condom use, age differences in the partnership, where they met and how soon after meeting they had sex.
Of the 11,161 respondents, 9,598 reported a total of 15,488 heterosexual partnerships in the past year. A higher proportion of men's partnerships were described as "not regular" - 39.1% compared to 20% of women's partnerships; while a higher proportion of women's partnerships were marriages or cohabitations - 55.2% versus 38.9% of men's partnerships. Men had sex sooner after first meeting a partner than women, with one in five men reporting sex within 24 hours of meeting their partner, compared with one in ten of women.
Condoms were used at last sex in 37.1% of men's and 28% of women's partnerships. Overall, half of all new partnerships involved condom use at first sex (55.3%), but this declined with age; for instance, 68% of men and 67.4% of women aged 16-19 used a condom at first sex, but only 38.1% of men and 28.8% of women aged 35-44 year-old did.
Dr Mercer said: "For some people not using condoms may be due to being or trying to become pregnant, but this is a less likely explanation for partnerships described as 'not regular', and it is therefore worrying that condom use was reported at last sex in just half of such partnerships. However, of greater concern was our finding that half of new partnerships did not use condoms at first sex, even when this was a non-regular partner, and condoms were not used in one-third of cases when first sex was within 24 hours of first meeting."
Rates of STIs are on the increase. In the UK, the Health Protection Agency reported a rise of six per cent in the total number of new STIs diagnosed in 2007 compared to 2006. Other research published this year reported that STI diagnosis rates in people aged 45 and older attending clinics in the West Midlands more than doubled between 1996 and 2003 .
"Our finding that condom use at first sex declined with increasing age is of concern," said Dr Mercer. "Although a disproportionate amount of partnerships are formed among people in their teens and 20s, the fact is that about 45% of marriages are now expected to end in divorce, which means that the 'population attributable risk' of partnership formation by those in their 30s and 40s will increase. Indeed, increasing rates of STIs diagnosed among those in their 30s and 40s suggest that interventions that promote consistent condom use with new partners are urgently required, not just for young people as has been the focus recently, but for people in their 30s and 40s and older who are increasingly forming new partnerships."
The researchers also found that where there was a large age difference between partners of five years or more, and regardless of whether it was the man or the woman that was older, partners were less likely to use condoms at first sex compared to partnerships that were closer in age: 44.1% versus 60.8%.
Dr Mercer said: "This may reflect unequal power relations, so that younger people in such partnerships may not have the necessary communication and negotiation skills to ensure safe sex with older partners. This finding may have implications for sex and relationship education and counselling. Improving negotiation skills for women and men may facilitate communication and, in turn, increase the likelihood of condom use in such partnerships."
She concluded: "It is important to understand the characteristics of sexual partnerships to improve our understanding of STI transmission and to ensure that health promotion messages are appropriately targeted and delivered. This paper helps us to better understand the population of partnerships (rather than the population of individuals) in terms of the large proportion of partnerships that are casual and why they are at increased risk of STIs."
 Who has sex with whom? Characteristics of heterosexual partnerships reported in a national probability survey and implications for STI risk. International Journal of Epidemiology. Published online under advance access. doi:10.1093/ije/dyn216.
 "Trends in sexually transmitted infections (other than HIV) in older persons: analysis of data from an enhanced surveillance system". Bodley-Tickell AT, Olowokure B, Bhaduri S et al. Sexually Transmitted Infections 2008; doi:10.1136/sti.2007.027847.