**4. Conclusions**

of São Paulo (11.96 and 0.36‰) [22]. These rates are slightly lower than results from developed countries (e.g., [69]). The increase in the twinning rate over the years was 14.54%, from 8.80‰ in 2002 to 10.08‰ in 2013, a modest increase compared to the 30.8% in São Paulo [22]. The decade time period positively predicted, albeit weakly, twin birth rates, explaining 3% of the variance. It also negatively predicted singleton rates, explaining 2% of

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

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

Among the five major regions of Brazil (North, Northeast, Central-West, South, and Southeast), we found a small, but significant variation in twinning rates. **Table 1** presents maternity rates for singletons, twins, and higher order multiple births divided by Brazilian regions. In general, the developed areas (e.g., Southeast) presented higher twinning and multiple birth rates than developing ones (e.g., North). This is in line with the finding of the São Paulo city districts. Similarly to the possible effects of higher maternal age, higher access to assisted reproduction technologies, and higher nutritional diversity and abundance may contribute to the higher twinning rates in the Southeast and South developed regions, yielding regional discrepancy. Furthermore, the ethnic composition may also play a role. In the North of Brazil, there is the highest proportion of indigenous intermixed individuals within the population (up to 32%), while in other regions it ranges from 11 to 16% [79]. Brazilian native Americans are closely

**Mean Singleton rates (‰) Twins rates (‰) Higher order rates (‰)**

Southeast 988.08 10.34 0.25 South 989.32 10.06 0.23 Central-West 990.70 9.05 0.24 Northeast 988.54 8.68 0.23 North 991.30 7.32 0.14

**Table 1.** Maternity rates of singleton, twin and higher order births per Brazilian regions.

the variance, with no effect on multiple birth rates.

38 Multiple Pregnancy - New Challenges

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

results from São Paulo [22, 56, 71] and from other countries, such as Spain [70].

strong increase in twinning and multiple births in women near menopause.

**3.2. Variation in twinning rates in different regions of Brazil**

The overall observed pattern of results linking increased twinning in mothers near menopause, from both decade-long large-scale populational studies, is consistent across São Paulo and all of Brazil, and agrees with the literature from Brazil [56, 71] and from other countries, for example, the Netherlands [58], and even with reports from traditional societies with natural reproduction, such as agricultural areas of Costa Rica [72], Gambia [55], eighteenth century Sweden [42], and seventeenth to eighteenth century French Canadian immigrants [41]. This convergence of results supports both evolutionary hypotheses about twinning: it points to the existence of an age-dependent evolved mechanism of twinning as an 'all-or-nothing' last chance strategy for reproduction near menopause. The estimated twinning rate per zygosity allowed us to closely test both the insurance ova hypothesis and the relaxed-screening hypothesis. In at least seven different populations worldwide [22, 70, 74–77], both DZ and MZ twinning rates increased in mothers near menopause; thus, both evolutionary theories received support, because it identifies polyovulation and relaxed screening as possible underlying mechanisms of increased twin births in women of higher age. The appreciation that also MZ twin births cross-culturally increases with mothers' age is an underappreciated pattern in the literature. Again, as stated earlier, these results require additional examination with respect to MZ twinning which has its origins explained by biological events that do not apply to DZ twinning (e.g., zygotic division).

Additionally, our results showed that higher socioeconomic conditions are related to higher twinning rates. This was found in both São Paulo and the entire country of Brazil, and corroborates the literature that has already shown a link between higher socioeconomic status and increased twinning rates in Brazil [78, 82] and other countries, for example, in Greece [83]. This finding also agrees with the literature on nonhuman primates that shows relatively higher twinning in captive, safer, and abundant conditions [1] and in species with more diverse nutritional intake [20].

This chapter reflects the importance of integrating factors and patterns from studies conducted on nonhuman species, particularly primates, with the findings traditionally focused on humans. We have attempted the first step toward bringing the comparative approach into light in order to promote deeper understanding of demographic data on twinning and higher order maternities. The comparative approach can offer insights and increase our understanding of both commonalities and specificities of the human case. Along the same lines, we have stressed the importance of considering proximate factors, such as genetic, physiological, ontogenetic and contextual variables, and distal factors, such as ancestral selective pressures, and evolutionary reasoning. Both fields have a great deal to gain with a more integrated approach.
