The number of unmarried couples in which one partner carries the hepatitis B virus rose in the past decade, according to a new survey. The Family Planning Association studied 10,000 couples in the last two decades and found that 6.5 per cent were carriers last year, compared with 5.1 per cent in 2001. This prompted the association to warn young adults and couples to undertake pre-pregnancy check-ups, as once they became sexually active they would be more prone to hepatitis B infection. The warning came as another new study by the Department of Health identified adults aged in their late 20s and early 40s accounting for more than half the reported cases of acute hepatitis B infections in Hong Kong. 'It's worth noting these people [young adults], at the prime age for marriage and reproduction, have a higher chance of contracting this infectious disease,' said Dr Susan Fan Yun-sun, the association's executive director. 'Certain practices popular among the younger generation, such as promiscuity, could even increase the chance of being infected by hepatitis B, which is transmitted through sexual contact.' Hepatitis B remains a sexual and reproductive health concern as the virus is transmitted through blood or body fluids, including sharing of needles, toothbrushes and shavers. However the main modes of transmission were from mother-to-baby during birth and practising unsafe sex, Fan said. 'During pregnancy, there is a vigorous exchange of body fluids between mother and the baby, which is not protected by any immune system,' said Dr Nancy Leung Wai-yee, the association's medical consultant. Despite the government's efforts since 1988 to offer free vaccinations for every newborn, medical research showed hepatitis B infection - estimated at up to 300,000 cases or one in every 12 adults in the city - was still a public health issue. More than 90 per cent of liver cancer cases in Hong Kong, the second-largest killer for men, developed from hepatitis B infections. However, by taking extra precautions, carriers could still give birth to uninfected babies. Fan urged carrier mothers and their partners to have pre-pregnancy check-ups and get vaccinated as soon as possible, as this could offer up to 90 per cent protection to the newborn. Infants born to carrier mothers should receive hepatitis B immunoglobulin vaccinations, in addition to the ordinary hepatitis B vaccine within 24 hours of birth.