Early sexual initiation among adolescents remains a global public health concern with far-reaching implications for sexual and reproductive health. Defined as sexual debut before the age of 15–16, early initiation is consistently associated with heightened vulnerability to unintended pregnancies, unsafe abortions, and sexually transmitted infections, particularly HIV (Santelli et al., 2017; CDC, 2022). In sub-Saharan Africa, and Nigeria in particular, the intersection of early sexual activity, adolescent pregnancy, and mother-to-child transmission (MTCT) of HIV presents urgent challenges (UNAIDS, 2020; NACA, 2021). Despite the existence of sexual health education programs and PMTCT services, adolescents continue to face barriers in access, utilization, and adherence, leaving significant gaps in prevention (WHO, 2021; UNICEF, 2021). This study investigates the prevalence of early sexual initiation among adolescent girls, its association with adolescent pregnancy and HIV infection, and its contribution to MTCT risk. It further evaluates the awareness and utilization of PMTCT services, explores the impact of comprehensive sexual health education (CSE) on adolescent behavior, and identifies barriers to effective HIV prevention in the adolescent population. A mixed-methods design was employed, combining quantitative surveys with qualitative interviews to ensure depth and breadth of analysis. A total of 500 adolescent girls aged 13–19 years participated in structured surveys capturing sexual history, contraceptive use, and HIV testing. Additionally, 30 in-depth interviews with adolescent mothers provided insights into lived experiences, stigma, cultural influences, and service barriers. Quantitative data were analyzed using descriptive statistics, chi-square tests, and logistic regression to establish associations, while qualitative data were subjected to thematic analysis to capture context-specific narratives. Findings revealed that 42% of participants reported early sexual initiation, with a mean debut age of 14.7 years. Early initiation was significantly associated with adolescent pregnancy (OR = 2.15, 95% CI: 1.45–3.19, p < 0.001) and HIV infection (OR = 1.89, 95% CI: 1.12–3.17, p < 0.05). Among adolescent mothers, only 55% reported consistent utilization of PMTCT services, with stigma, lack of transportation, and fear of disclosure identified as key barriers. Exposure to comprehensive sexual health education was linked to delayed sexual debut (mean 16.1 vs. 14.3 years, p < 0.001), higher condom use (62% vs. 38%, p < 0.001), and a 35% reduction in HIV risk behaviors (OR = 0.65, 95% CI: 0.44–0.96, p < 0.05) (Kirby, 2019; Kirby & Laris, 2019). The study underscores the strong link between early sexual initiation, adolescent pregnancy, and HIV risk, with profound implications for MTCT. Despite global advances, adolescent mothers remain underserved by PMTCT programs, with structural, cultural, and psychosocial barriers limiting uptake (Okafor &Ogbo, 2018; Olukoya et al., 2017). Comprehensive sexual health education emerges as a protective factor, delaying sexual debut and promoting safer practices (UNESCO/WHO/UNFPA, 2018; Chika & Uche, 2020). To reduce adolescent HIV risk and prevent MTCT, health systems must prioritize youth-friendly PMTCT services, expand comprehensive sexual health education, and address socio-cultural barriers through community engagement. Policy reforms ensuring confidentiality, accessibility, and affordability of adolescent reproductive health services are essential (WHO, 2019; Patton et al., 2016).