Background Teenage pregnancy rates across the UK and other English-speaking high-income countries have fallen across recent decades. Britain’s rates fell from 46.9 pregnancies per 1,000 women aged under 18 in 1998 to 15.8 per 1,000 women in 2019. The reasons are not well understood, hampering research and the development of policy. We used systematic review methods to identify plausible hypothesised causes of this decline in pregnancy rates.

Methods From the sexual health literature, we constructed an initial tree diagram of interacting causal ‘branches’. Branches included teenage pregnancy interventions and wider environmental changes which may have influenced pregnancy rates. From this causal tree we developed a search strategy, combining terms to sensitively search for ‘adolescent’ or ‘teenage’ populations with a ‘pregnancy’ or ‘conception’, alongside terms relating to ‘contraception’ or action to ‘reduce’ risk or ‘prevent’ pregnancies. We searched four databases for reviews published since 1990 assessing evidence for each of the branches’ contributions to falling rates within the UK and in similar high-income European or English-speaking countries. We extracted data on the strength of evidence for existing branches and assessed evidence for additional branches. Reviews were graded according to risk of bias. Evidence across reviews was synthesised – prioritising high-quality reviews that used systematic search and synthesis methods – to give overall assessments of the certainty of 1) effectiveness of an exposure in reducing pregnancy rates, using modified Bradford Hill criteria; and 2) reach across whole populations of the exposure of interest. We used these combined scores to assess the causal contribution of each branch to observed falling rates.

Results We identified 89 reviews from 631 articles found through searches and citations. 62 were rated high-quality. Included reviews presented evidence assessing the contribution of 20 causal branches. Evidence for teenage pregnancy interventions was more common than evidence for environmental changes. We eliminated two branches which had been extensively reviewed but for which evidence was consistent with a null effect and added seven novel branches. We retained 13 plausible contributors to falling teenage pregnancy rates in the updated causal tree. Two branches – changing contraceptive technologies and policies promoting increased educational involvement – were rated as having ‘strong evidence’. Two branches were not assessed in any reviews and were retained out of caution; the remaining nine included branches presented ‘weak evidence’ of their effect on rates.

Conclusion The evidence-based causal tree provides a basis for both further investigation of hypothesised causes and development of effective interventions.