Teenage pregnancy is extremely common in South Africa. Whilst its 'problematic' nature is a subject of debate, it reflects a pattern of sexual activity which puts teenagers at risk of HIV. Currently one in five pregnant teenagers is infected with the virus. This creates a new imperative to understand teenage pregnancy and the pattern of high risk sexual activity of which it is one consequence. This was an exploratory study undertaken to investigate factors associated with teenage pregnancy amongst sexually active adolescents in an urban and peri-urban context. The study used a matched case control design, with 191 cases and 353 school or neighbourhood, age-matched controls. Subjects were under 19 years and recruited from township areas of Cape Town. A structured questionnaire was used to obtain information on socio-economic factors, contraceptive knowledge and use, and sexual behaviour. Conditional logistic regression was used to analyse the relationship between teenage pregnancy and the factors investigated. The results presented focus on relationship dynamics and their association with the risk of pregnancy. Both groups of teenagers had been dating for a mean of two and a half years and about half were still with their first sexual partner. The partners of the pregnant teenagers were significantly older, less likely to be in school and less likely to have other girlfriends. The pregnant teenagers were significantly more likely to have experienced forced sexual initiation and were beaten more often. They were much less likely to have confronted their boyfriend when they discovered he had other girlfriends. Multiple modelling shows that both forced sexual initiation and unwillingness to confront an unfaithful partner are strongly associated with pregnancy and also related to each other. We argue that the associations are mediated through unequal power relations within the relationship which are reinforced by violence. We further discuss indicators of greater intimacy within relationships of the pregnant teenagers which may suggest that more of the pregnancies were wanted than was suggested. Both of these conclusions pose critical challenges for health promoters.