Interaction of age with scratch-effect on live birth, combined representation of IPD participant-level analysis, IPD study-level analysis, and analysis within each trial. Plot of the interaction between age and the scratch-effect on live birth, at participant- and study-level. The red line represents the log-OR of scratch versus control for live birth as a function of mean participant age (one-stage participant-level analysis), and the green line represents the meta-regression of per-trial treatment effect estimates versus mean participant age (two-stage study-level analysis). The dashed black lines represent the interaction between age and treatment effect on live birth within each trial (two-stage analysis).