**3. Twinning rates across the entire country of Brazil**

addresses at the time of delivery, we found a large variance in the twinning rates among the 31 subregions of São Paulo: from 9.85 (district Itaim Paulista) to 24.32‰ (district Pinheiros). **Figure 4** shows the overall rate of twins' geographical distribution between 2003 and 2015 by the 31 subregions of the city. In **Figure 4**, we clearly show that in the central regions there are more twins born than in peripheral districts. This centralized distribution does not simply reflect the places where most hospitals are available, because we accessed the mothers' addresses and not the address of the maternity hospitals. Thus, this indicates that a real

**Figure 3.** Dizygotic (DZ) and monozygotic (MZ) twin rates per 1,000 births as a function of mother's age; from [22].

There was a pronounced positive correlation between the average income of each city district according to the 2010 Demographic census from Brazilian Institute of Geography and Statistics (IBGE) and the average twinning rates during the period from 2003 to 2015. The same result was found for the higher order multiple birth rates. This indicates that, in richer regions, there is a higher chance of twin and multiple births. This new finding from the USP Twin Panel can explain the centralized distribution of higher twinning rates shown in **Figure 4**. Many factors could be interacting to produce this result. Women with higher socioeconomic status tend to study longer and delay reproduction; thus, having offspring in higher age increases the chance of multiple pregnancies. Also those women can afford assisted reproduction technologies which increase the chance of multiple pregnancies. Moreover, those women have a richer and more diverse nutritional diet which, as found for nonhuman primates [1, 20], could increase the chance of multiple pregnancies. Colletto et al. [78] also found increased twinning rates as a function of women's higher socioeconomic

demographic factor is driving this distribution.

36 Multiple Pregnancy - New Challenges

Most recently, the USP Twin Panel (*Painel USP de Gêmeos*) has expanded the investigation of twinning rates to the entire country of Brazil between the years of 2002 and 2013. The data were drawn from TABNET within DATASUS, an official public governmental database and transformed into rates of maternities the same way it was done for the city of São Paulo [22]. A multivariate general linear model was used to explore the effects of region, maternal age, and time period on singleton, twin and higher order multiple birth rates. We obtained a total of 35,051,790 maternities between 2002 and 2013, 329,006 twinning maternities and 8,005 higher order maternities. Considering all regions of Brazil, the overall average rate of twins was 9.39‰ and higher order births were 0.23‰. These data show that the Brazilian countrywide twinning and multiple birth rates are lower than the twinning rates in the city 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 the variance, with no effect on multiple birth rates.

related to Asiatic populations [80] that have the lowest twin rates compared to other ethnicities [39, 81]. It is thus possible that Asian ancestry may underlie the relatively lower twinning rate in the northern region of Brazil. Future studies should further explore the influencing

Twinning as an Evolved Age-Dependent Physiological Mechanism…

http://dx.doi.org/10.5772/intechopen.79907

39

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

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.

factors among the regions.

diverse nutritional intake [20].

**4. Conclusions**
