Background The overall goal of this study was to determine the causative factors for pregnancy status in adolescent girls in two communities in the Lagos Island local government area. Methods A mixed methods (quantitative and qualitative) study reviewing routine health facility antenatal care (ANC) records and conducting focus group discussions among 46 pregnant adolescents, exploring their views about sex, contraception, pregnancy and sexual and reproductive health (SRH) services, was carried out. Key informant interviews were also carried out among healthcare workers and community members to assess their perceptions of adolescents' SRH problems. Results Five percent of those accessing ANC services were adolescents. Pregnant adolescents were found to access health services at later stages of their pregnancies due to the shame and stigma associated with their condition. The presence of morbidity in the form of anaemia (33%) and HIV (2.4%) was also found in this population. Social factors such as peer pressure and the desire to develop or maintain a relationship were found to be the major reasons for initiation of sexual activity by the adolescents. There was generally poor knowledge and utilisation of contraceptives, leading to unprotected sex and, thus, unintended pregnancies. Conclusion SRH information and services should be made readily available to adolescents at all levels of care.