Birth rate of Twins and Multiples according to maternal age in the city of São Paulo from 2003 to 2014
This research aimed to study the rate of twins in the city of São Paulo. We pioneered the birth data of all twins and multiples (triplets or more) between 2003-2014 in 140 hospitals in the city (56 public and 84 private). Several factors that affect the birth rate of twins and multiples have already been studied in other countries, among them are maternal age, higher BMI (body mass index), the degree of certain types of pollutants in the atmosphere and also assisted reproduction, among other factors.
The results indicated that there was an increase in the number of births of twins in the city of São Paulo between 2002 and 2014 of about 30% (from 10.1 to 13.3‰), at the same time as there was a decrease in the number of births of singletons and multiples. The overall rate of twins in the city of São Paulo in this period was 11.96‰ (being 4.4‰ monozygotic and 7.15‰ dizygotic) and 0.36‰ multiples.
As with other studies done previously, our results indicated the existence of a physiological mechanism linked to higher maternal age that leads to an increase in the birth rate of twins and multiples, but not the birth of single-children. In addition, the evaluated data demonstrated that there is a higher chance of older women having fraternal twins (dizygotics) compared to the chance of having identical (monozygotic) twins, even though this difference is small.
One of the explanations for a higher rate of twins in older women is that at the end of their reproductive age they ovulate more egg cells, which leads to a greater chance of twin pregnancies. Another explanation is that older women resort to fertilization techniques, increasing the likelihood of twins and multiples being born. To read more about our research, click below.