Infertility, infertility treatment and twinning: the Danish National Birth Cohort

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 infer-tile 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 twi