Hum. Reprod. Advance Access originally published online on January 4, 2007
Human Reproduction 2007 22(4):1086-1090; doi:10.1093/humrep/del495
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Infertility, infertility treatment and twinning: the Danish National Birth Cohort
1 The Danish Epidemiology Science Centre, Department of Epidemiology, Institute of Public Health, University of Aarhus, Aarhus C, Denmark 2 Epidemiology Branch, National Institute of Environmental Health Sciences, Department of Health and Human Services, National Institutes of Health, Research Triangle Park, NC, USA 3 Perinatal Epidemiological Research Unit, Department of Gynaecology and Obstetrics, Aarhus University Hospital, Skejby, Aarhus N, Denmark 4 Center for the Prevention of Congenital Malformations, Institute of Public Health, University of Southern Denmark, Odense, Denmark 5 Department of Epidemiology, School of Public Health, UCLA, Los Angeles, CA, USA
6 To whom correspondence should be addressed at: The Danish Epidemiology Science Centre, Department of Epidemiology, Institute of Public Health, University of Aarhus, Vennelyst Boulevard 6, DK 8000 Aarhus C, Denmark. Tel: +45 89426074; Fax: +45 86131580; E-mail: zjl{at}soci.au.dk
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 TTP 
12 months, 5838 infertile couples who conceived naturally with 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.
Key words: infertility/infertility treatment/time to pregnancy/twinning/zygosity
Submitted on July 4, 2006; resubmitted on October 17, 2006; resubmitted on November 21, 2006; accepted on November 29, 2006.
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