Individual trajectories of fertility with age highlight the costs of reproduction in a pre-industrial population.

Evolutionary theories predict that early-life reproductive effort should be favoured by selection at the cost of faster reproductive ageing, but empirical support for this trade-off remains mixed. Previous studies typically examined whether the rate of reproductive ageing varied among groups of individuals differing in early-life reproductive allocation. However, this approach assumes that all individuals have the same age at the onset of reproductive ageing. By analysing annual birth probabilities among women aged 16-50 years in a pre-industrial population of Nouvelle-France (Quebec, Canada), we showed reproductive ageing displayed high variability between mothers both in onset (n = 7346) and rate (n = 2663). Age at the onset of reproductive ageing varied nonlinearly with the number of children produced during early life. Regardless of sex, the first child born no later than 28 years advanced the onset of reproductive ageing by 914 ± 25 days and each additional child by 220 ± 7 days. An increasing number of children born by age 28 also accelerated the rate of reproductive ageing. These results highlight the value of individual trajectories in reproductive ageing studies and may improve understanding of how early-life reproduction influences late-life reproduction, especially in low-income countries with high birth rates.