1 Institute of Epidemiology and Social Medicine, Faculty of Health Sciences, Aarhus University, Aarhus University2 Department of Public Health - Department of Epidemiology, Department of Public Health, Health, Aarhus University3 National Institute of Environmental Health Sciences, National Institutes of Health4 Perinatal Epidemiological Research Unit, Department of Gynaecology and Obstetrics, Aarhus University Hospital5 Center for the Prevention of Congenital Malformations, Institute of Public Health, University of Southern Denmark6 Department of Public Health - Department of Epidemiology, Department of Public Health, Health, Aarhus University
BACKGROUND We have previously observed that an increasing time to pregnancy (TTP) is associated with a reduced frequency of twin deliveries in couples not receiving infertility treatment. By using updated information, we assessed the frequencies of dizygotic (DZ) and monozygotic (MZ) twin deliveries as a function of infertility (TTP>12 months), as well as infertility treatment. METHODS From the Danish National Birth Cohort (1997-2003), we identified 51 730 fertile couples with TTPwith TTP>12 months and 5163 infertile couples who conceived after treatment. Information on zygosity, available for part of the cohort (1997-2000), was based on standardized questions on the similarities between the twins at the age of 3-5 years. RESULTS Compared with fertile couples, the frequency of DZ twin deliveries was lower for infertile couples conceiving naturally (odds ratio 0.4, 95% confidence interval 0.2-0.7) and was much higher for infertile couples conceiving after treatment (17.3, 14.4-20.7). The frequency of DZ twin deliveries decreased with TTP in untreated couples, whereas the frequency of MZ twin deliveries remained constant. CONCLUSIONS The frequency of DZ twin deliveries decreased with TTP and substantially increased with infertility treatment, whereas MZ twin deliveries remained substantially unchanged.