Women's multi-partner behavior, multi-partner fertility, and pregnancy outcomes: findings from the 2004 Pelotas Birth Cohort.

STUDY QUESTION: Do women with multi-partner fertility or multi-partner behavior conceive more often than women with a single partner? SUMMARY ANSWER: Women with multi-partner behavior conceived more frequently and had more children than non-multi-partner women and multi-partner fertility women. WHAT IS KNOWN ALREADY: Some women experience having biological children with more than one partner: those women are considered as multi-partner fertility. Women with multi-partner fertility have more children and are substantially less likely to have planned their first birth. Individuals with multi-partner fertility become parents at a younger age, largely with unintended first births, and often do so outside of marriage, compared to parents with two or more children from only one partner. Unmarried women, particularly, are at greater risk of having unintended births. Studies are still scarce and there is a need to assess the contribution of women's multi-partners fertility and multi-partner behavior to family composition, particularly in low- and middle-income countries. STUDY DESIGN, SIZE, DURATION: This longitudinal birth cohort study evaluated 1215 mothers whose children belonging to the 2004 Pelotas Birth Cohort were their first pregnancy, and who attended the perinatal, 48-month, 6-year, and 11-year follow-ups. Information was obtained from responses to a questionnaire. The number of years at risk of having children was treated as the exposure, and woman's multi-partner behavior and multi-partner fertility, dichotomized as 'Yes' or 'No', were considered endogenous treatment variables. PARTICIPANTS/MATERIALS, SETTING, METHODS: Data from mothers with a first pregnancy, and with information available from the perinatal, 48-month, 6-year, and 11-year follow-ups, were evaluated. The exposures studied were women's multi-partner behavior and multi-partner fertility (i.e. conceiving/giving birth), and the outcomes evaluated were the number of pregnancies, the number of children currently alive, and experience of unintended pregnancies from the birth of the child belonging to the 2004 birth cohort until 11 years later. Crude and adjusted risk ratios (RRs) were estimated through Poisson regression with endogenous treatment effects, robust standard errors, and their respective 95% CI. MAIN RESULTS AND THE ROLE OF CHANCE: Multi-partner behavior women had 16% (RR 1.16; 95% CI: 1.08-1.25) and 11% (RR 1.11; 95% CI 1.03-1.19) greater risk of having a new pregnancy and having more children alive, respectively, than those with non-multi-partner behavior. Women with multi-partner fertility had a 23% (RR 1.23; 95% CI: 1.11-1.37) and 20% (RR 1.20; 95% CI: 1.08-1.33) higher risk of having a new pregnancy and having more children alive, respectively, than single-partner fertility mothers. Women who had multiple partners (i.e. behavior), as well as those with multi-partner fertility, showed a lesser proportion of unintended pregnancies when compared to the non-multi-partner ones (34.08%; 95% CI: 28.12-40.60 vs 36.17%; 95% CI: 31.93-40.63), compared to their counterparts' single partners fertility (33.16%; 95% CI: 26.83-40.17 vs 36.26%; 95% CI: 31.85-40.92), although these findings were not statistically significant. LIMITATIONS, REASONS FOR CAUTION: The mothers who were not included in the study owing to missing data for some of the follow-up had 5-11 years of education, a low socio-economic level, and were younger, thus the number of pregnancies may be underestimated because these groups presented a high number of pregnancies and children alive. We did not have information about the complete woman's conjugal history. Therefore, misclassification error of the exposure may be present and, consequently, the measures of association may be underestimated. Furthermore, this study was not truly representative of the Pelotas study female population. WIDER IMPLICATIONS OF THE FINDINGS: In this study of multi-partner behavior and fertility, women who have multiple partners may be less likely to get married and have a stable partner. Compared to single-partner women, multi-partner fertility and multi-partner behavior women may predominantly become pregnant for the purpose of having children, rather than accidentally. STUDY FUNDING/COMPETING INTEREST(S): This article is based on data from the study 'Pelotas Birth Cohort, 2004' conducted by the Postgraduate Program in Epidemiology at the Universidade Federal de Pelotas, with the collaboration of the Brazilian Public Health Association (ABRASCO). From 2009 to 2013, the Wellcome Trust supported the 2004 birth cohort study. The World Health Organization, National Support Program for Centers of Excellence (PRONEX), Brazilian National Research Council (CNPq), Brazilian Ministry of Health, and Children's Pastorate supported previous phases of the study, and also was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior-Brasil (CAPES)-Finance Code 001. The authors declare that the supported agencies have no role in any step of performing this study. No conflicts of interest exist. TRIAL REGISTRATION NUMBER: N/A.