Fig. 4. Relationship between maternal age, parity and twinning status and the three life-history events describing maternal reproductive life.

a The probability that a mother keeps reproducing after a given parity (parity progression) was lower following the birth of twins than following the birth of a singleton (n = 105,833 births in total, from 21,290 mothers). Marginal predictions for the probability of parity progression are shown in black with open symbols, with scale on the left y-axis, and were computed given a parity and twinning status as average predictions over the empirical distribution of maternal age at each parity. The mean of maternal age for each parity is shown by the grey line with filled symbols, with the scale on the right y-axis. b The duration between two consecutive births (interbirth interval) increased with parity (colour) but slightly decreased after the birth of twins vs singleton (line type) or with maternal age (x-axis) when parity is held constant (n = 84,543 births in total, from the 18,520 mothers who had at least two births). c Mothers presented the highest risk of twinning (n = 105,833 births in total, from 21,290 mothers) during their mid and late thirties, as well as at parity 1 (purple line). For (b, c), marginal predictions are shown as curves and are computed for maternal age and parity values spanning the 95% central ranges of the observed distributions of these variables. Model summary statistics are provided in Supplementary Tables 4–6.