**3.1. Twinning as a function of mothers' age in the entire country of Brazil**

Our results showed that maternal age strongly and positively predicted twin and multiple birth rates, explaining 56 and 21% of the variances, respectively. It also negatively predicted singleton birth rates, explaining 62% of the variance. These results are in agreement with the results from São Paulo [22, 56, 71] and from other countries, such as Spain [70].

The finding that the higher maternal age predicts twin and multiple births corroborates the evolutionary hypotheses, the insurance ova hypothesis [57, 59] and the relaxed-screening hypothesis [60, 61], pointing to the existence of an age-dependent mechanism that leads to a strong increase in twinning and multiple births in women near menopause.
