Abstract STUDY QUESTION Is parents’ age at birth associated with daughters’ fecundability? SUMMARY ANSWER Daughters born to mothers <25 years or fathers ≥35 years had slightly lower fecundability. WHAT IS KNOWN ALREADY Two recent studies reported lower fecundability in women born to mothers <20 years, which may be partly due to daughters of young mothers being less likely to plan their pregnancies. STUDY DESIGN, SIZE, DURATION A retrospective cohort study of 58 496 pregnancy planners (4290 of whom conceived with treatment) and 14 194 non-planners enrolled in the Norwegian Mother, Father and Child Cohort Study (MoBa) between 2000 and 2008, linked with the Medical Birth Registry of Norway. PARTICIPANTS/MATERIALS, SETTING, METHODS Participants were born in Norway between 1967 and 1990. We estimated fecundability ratios (FRs) and 95% CI as a function of both parents’ (F1) age at the daughter’s (F2) birth among non-treated planners and the relative risk of time to pregnancy (TTP) ≥12 months or treatment among all planners. We explored whether daughters of young mothers were under-represented among planners, compared with the underlying population. Finally, we estimated FRs after adding non-planners, randomly assigned to conceiving in the first cycle with probabilities of 0.60 and 0.70. MAIN RESULTS AND THE ROLE OF CHANCE For both mother and father, the reference category was 25–29 years. Fecundability was slightly lower among daughters of older fathers (FRs (95% CI): 0.95 (0.92, 0.98) for F1 father’s age 35–39 years and 0.93 (0.89, 0.97) for ≥40 years) and daughters of young mothers (0.92 (0.89, 0.96) for F1 mother’s age <20 years and 0.97 (0.95, 0.99) for 20–24 years). Results were similar for the composite outcome TTP ≥ 12 months or treatment, although driven by TTP ≥ 12. Compared with Norwegian-born women with ≥1 pregnancy, planners born to mothers <20 years were underrepresented. Including non-planners with very high fecundability weakened the association with mother’s age <20 years. LIMITATIONS, REASONS FOR CAUTION This was a pregnancy cohort with retrospectively reported information on planning and TTP. Selection bias appears unlikely to fully explain the association with mother’s age <20 years. WIDER IMPLICATIONS OF THE FINDINGS Daughters of young mothers or older fathers may have slightly lower fecundability. If corroborated, the finding about older paternal age is relevant, given the widespread tendency to delay childbearing. STUDY FUNDING/COMPETING INTEREST(S) This work was partly funded by the Research Council of Norway (project no. 320656), and through its Centres of Excellence funding scheme (project no. 262700). M.C.M. has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement no. 947684). No competing interests. TRIAL REGISTRATION NUMBER N/A.