Abstract: We included 53 RCTs that enrolled 105,368 adolescents. Participants were ethnically diverse. Eighteen studies randomised individuals, 32 randomised clusters (schools (20), classrooms (6), and communities/neighbourhoods (6). Three studies were mixed (individually and cluster randomised). The length of follow up varied from three months to seven years with more than 12 months being the most common duration. Four trials were conducted in low- and middle- income countries, and all others were conducted in high-income countries. Multiple interventions Results showed that multiple interventions (combination of educational and contraceptive-promoting interventions) lowered the risk of unintended pregnancy among adolescents significantly (RR 0.66, 95% CI 0.50 to 0.87; 4 individual RCTs, 1905 participants, moderate quality evidence. However, this reduction was not statistically significant from cluster RCTs. Evidence on the possible effects of interventions on secondary outcomes (initiation of sexual intercourse, use of birth control methods, abortion, childbirth, sexually transmitted diseases) was not conclusive. Methodological strengths included a relatively large sample size and statistical control for baseline differences, while limitations included lack of biological outcomes, possible self-report bias, analysis neglecting clustered randomisation and the use of different statistical tests in reporting outcomes. Educational interventions Educational interventions were unlikely to significantly delay the initiation of sexual intercourse among adolescents compared to controls (RR 0.95, 95% CI 0.71 to 1.27; 2 studies, 672 participants, low quality evidence). Educational interventions significantly increased reported condom use at last sex in adolescents compared to controls who did not receive the intervention (RR 1.18, 95% CI 1.06 to 1.32; 2 studies, 1431 participants, moderate quality evidence). However, it is not clear if the educational interventions had any effect on unintended pregnancy as this was not reported by any of the included studies. Contraceptive-promoting interventions For adolescents who received contraceptive-promoting interventions, there was little or no difference in the risk of unintended first pregnancy compared to controls (RR 1.01, 95% CI 0.81 to 1.26; 2 studies, 3,440 participants, moderate quality evidence). The use of hormonal contraceptives was significantly higher in adolescents in the intervention group compared to those in the control group (RR 2.22, 95% CI 1.07 to 4.62; 2 studies, 3,091 participants, high quality evidence) A combination of educational and contraceptive-promoting interventions appears to reduce unintended pregnancy among adolescents.  Evidence for programme effects on biological measures is limited. The variability in study populations, interventions and outcomes of included trials, and the paucity of studies directly comparing different interventions preclude a definitive conclusion regarding which type of intervention is most effective

Plain-language summary: This review included 53 randomised controlled trials comparing these interventions to various control groups (mostly usual standard sex education offered by schools). The search for trials was not limited by country, though most of the included trials were conducted in high-income countries, with just four trials in middle- and low-income countries, mainly representing the lower socio-economic groups. Interventions were administered in schools, community centres, healthcare facilities and homes. Meta-analysis was performed for studies where it was possible to extract data. Only interventions involving a combination of education and contraception promotion (multiple interventions) was seen to significantly reduce unintended pregnancy over the medium-term and long-term follow-up period. Results for behavioural (secondary) outcomes were inconsistent across trials. Limitations of this review include reliance on programme participants to report their behaviours accurately and methodological weaknesses in the trials.