Relative risk of composite outcome (time to pregnancy ≥12 months or fertility treatment) among all planners and of time to pregnancy ≥12 months among untreated planners, by F1 parents’ age. Vertical lines drawn at 0.95 and 1.05 to facilitate visualization. RR, relative risk; TTP, time to pregnancy. ^aCrude percentage of outcome in each age category. ^bAdjusted for F2 birth order (first born or later) and whether the F1 mother had smoked while pregnant with F2.