Background Adolescent pregnancies present a great public health burden in Kenya and Sub-Saharan Africa (UNFPA, Motherhood in Childhood: Facing the challenge of Adolescent Pregnancy, 2013). The disenfranchisement from public institutions and services is further compounded by cultural stigma and gender inequality creating emotional, psychosocial, health, and educational problems in the lives of vulnerable pregnant adolescents (Int J Adolesc Med Health 15(4):321–9, 2003; BMC Public Health 8:83, 2008). In this paper we have applied an engagement interview framework to examine interpersonal, practical, and cultural challenges faced by pregnant adolescents. Methods Using a qualitative study design, 12 pregnant adolescents (ages 15–19) visiting a health facility’s antenatal services in Nairobi were interviewed. All recruited adolescents were pregnant for the first time and screened positive on the nine-item Patient Health Questionnaire (PHQ-9) with 16% of 176 participants interviewed in a descriptive survey in the same Kangemi primary health facility found to be severely depressed (Osok et al., Depression and its psychosocial risk factors in pregnant Kenyan adolescents: a cross-sectional study in a community health Centre of Nairobi, BMC Psychiatry, 2018 18:136 https://doi.org/10.1186/s12888-018-1706-y). An engagement interview approach (Social Work 52(4):295–308, 2007) was applied to elicit various practical, psychological, interpersonal, and cultural barriers to life adjustment, service access, obtaining resources, and psychosocial support related to pregnancy. Grounded theory method was applied for qualitative data sifting and analysis (Strauss and Corbin, Basics of qualitative research, 1990). Results Findings revealed that pregnant adolescents face four major areas of challenges, including depression, anxiety and stress around the pregnancy, denial of the pregnancy, lack of basic needs provisions and care, and restricted educational or livelihood opportunities for personal development post pregnancy. These challenges were related both to existing social and cultural values/norms on gender and traditional family structure, as well as to service structural barriers (including prenatal care, mental health care, newborn care, parenting support services). More importantly, dealing with these challenges has led to negative mental health consequences in adolescent pregnant girls, including feeling insecure about the future, feeling very defeated and sad to be pregnant, and feeling unsupported and disempowered in providing care for the baby. Conclusions Findings have implications for service planning, including developing more integrated mental health services for pregnant adolescents. Additionally, we felt a need for developing reproductive education and information dissemination strategies to improve community members’ knowledge of pregnant adolescent mental health issues. Electronic supplementary material The online version of this article (10.1186/s12905-018-0581-5) contains supplementary material, which is available to authorized users.