A retrospective study was undertaken of all women attending a sexual health service during their pregnancies in the year 2000 in order to find the prevalence of sexually transmitted infections (STIs) and other conditions commonly screened for in sexual health services among this population. Data relating to demographics, sexual health screen and infections diagnosed were collected. All new female registrations during the same period were additionally identified. The prevalence of disease in the two groups was compared. Data on 164 patients who attended during their pregnancies in the year 2000 were analysed. One STI was identified in 59 patients (36%), two infections in nine patients (5%) and three infections in three patients (2%). An increased prevalence of STIs was noted in those aged 25 and under, compared with those over 25 (P < 0.001). Prevalence of STIs increased with gestation (0.01 < P > 0.05). Prevalence of STIs was higher in pregnant women (63/164; 38.4%) compared with all new female registrations (1094/5273; 20.7%; P < 0.001) during the same period. Additionally, 8.5% of pregnant women had a negative screen compared with 20% of all new female cases. This study shows the prevalence of STIs to be significantly higher among the pregnant women as compared with all women attending. A trend towards more frequent occurrence of an STI was seen with increasing gestation and young age. Prospective studies are needed to verify these results among an unselected population of pregnant women.