This study showed that twins accounted for a small proportion of births in the 2004 and 2015 Pelotas Birth Cohorts (2.0% and 2.6%, respectively), but that their contributions to the prematurity rate, NICU admissions, and infant mortality increased in this 11- year period. In 2004, twin pregnancies accounted for 4.4% of preterm births, but by 2015 they had increased to 7.5% (p = 0.04), mainly with births < 34 weeks gestational age (increase of 122% from 2004 to 2015). Along with this change, there was an increase in the proportion of twins who required NICU admission (independently of birth order) and who died in the first year of life, especially among second-born twins. The contribution of twins to infant mortality was small, but it more than doubled in this period (from 4.0% in 2004 to 9% in 2015), which was due not only to the absolute increase in deaths among twins, but also to the decrease in the overall number of infant deaths in Pelotas during the same period.(29)
The study’s limitations include the lack of information on family history of multiple pregnancies and the small number of twin births in both cohorts, which affects the study’s power to identify associations between potential risk factors and the outcomes. The small sample size also prevented comparing perinatal outcomes and mortality between first- and second-born twins according to gestational age and type of delivery, which are important prognostic characteristics for twins’ survival, as reported by other authors.(11) Meanwhile, the use of two population-based birth cohorts from the same city, eleven years apart, with a low rate of attrition is a strength of this study. Data collected for each twin separately at the time of birth is also a strength mainly considering that information on second-born twins tends to be insufficient, especially when obtained from hospital medical records.(30) In addition, the mothers and twins were analyzed separately, and the first- and second-born twins were analyzed individually according to incidence of unfavorable perinatal outcomes and mortality in the first year of life. Although the prevalence and consequences of twin pregnancies may vary from place to place, the results of this study may be valid to other settings with similar socio-economic and maternal sociodemographic characteristics.
There was no difference in twinning rates between the two cohorts, but the rate in 2015 (1.4%) was higher than in the Brazilian state where Pelotas is located (Rio Grande do Sul) (1.2%) (p = 0.04) and in Brazil as a whole (1.13%) (p = 0.002) from 2011 to 2014.(4) This finding is consistent with the fact that the prevalence of some maternal factors associated with the occurrence of twin pregnancies, such as higher age and obesity,(20, 31) increased in Pelotas in the last three decades. The proportion of mothers 35 years or older increased from 9.9% in 1982 to 11.0%, 13.3%, and 14.7% in 1993, 2004, and 2015, respectively.(32) During the same period, the prevalence of pregestational maternal obesity (BMI ≥ 30 kg/m2) increased sharply, from 4.4% of mothers in 1982 to 4.9%, 9.0%, and 18.7% in 1993, 2004, and 2015, respectively.(33)
In both cohorts, there was no statistical difference in the multiple pregnancy rate according to maternal age or family income. However, in 2015, multiple pregnancies were more frequent in mothers 35 years or older (2.3%) and in families with monthly incomes between 6.1 and 10.0 minimum wage (2.8%). Older maternal age, together with genetic background and ethnicity, are known factors for multiple pregnancies.(1, 20, 30, 34) As for income, the current finding is consistent with the results of previous analyses in this cohort. Eighteen mothers in 2015 gave birth to 23 newborns conceived by assisted reproduction procedures (in vitro fertilization in 70.6% of the cases). Nine of the 23 children were newborns from multiple pregnancies.(35)
Although black ethnicity, multiparity, and low socioeconomic status have been associated with multiple pregnancies in other studies, particularly dizygotic twins,(4) we found no difference in these factors in our two cohorts. On the contrary, in the 2015 cohort, mothers with the highest family incomes showed higher than average twinning rates. In addition, a study that compared twinning rates in Pelotas from 1982 to 2015 showed a 220% increase among mothers with higher socioeconomic status and a 60% increase in white mothers, but no similar trend in brown or black mothers.(36) In our study, the small number of twin pregnancies may explain the lack of association between self-reported skin color and parity.
The estimated rate of dizygotic twins nearly doubled in Pelotas in the last decade. The factors associated with monozygotic and dizygotic twin pregnancies are different. While monozygotic pregnancies are mostly determined by genetic factors and in pregnancies resulting from in vitro fertilization, dizygotic pregnancies have more sociodemographic determinants, including ethnicity (more frequent in black women), multiparity, maternal age 35 years or older, low socioeconomic status, oral contraceptive use, family history, higher maternal height, obesity, and use of assisted reproduction techniques.(4, 31) However, the method we used to estimate the type of twin pregnancy is more robust and reliable when applied to larger databases.(37, 38)
The increase in prematurity among twins differs from the relative stability in the overall prematurity rate in births in Pelotas during this period (13.7% in 2004 and 13.8% in 2015).(27) Multiple pregnancies are the principal factor independently associated with spontaneous preterm births in Brazil,(39) and twin pregnancies are also risk factors for medically induced preterm births.(40)
In the two cohorts, all unfavorable perinatal outcomes were worse in second-born than first-born twins. However, contrary to reports by other authors,(6, 11–14) the differences were not statistically significant, probably due to the small sample size. Compared to singletons, the risk of second-born twins in the 2015 cohort dying in the post-neonatal period and in the first year of life was, respectively, eight and six times greater, indicating their greater vulnerability.