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BACKGROUND: More than 10% of babies are born to couples taking >1 year to conceive (a common definition of infertility). Some evidence indicates that such pregnancies are at increased risk of preterm delivery and other adverse birth outcomes, but the problem has rarely been addressed in large, longitudinal studies enrolling couples irrespective of infertility treatment. METHODS: We used data from the Danish National Birth Cohort: 55 906 singleton live births from women who reported their waiting time to pregnancy (TTP) and other covariates in an interview during the 2nd trimester of pregnancy. RESULTS: A TTP >1 year was associated with an increased risk of all outcomes studied, including preterm birth [odds ratios and 95% confidence intervals were 1.5 (1.2, 1.8) among primiparas and 1.9 (1.5, 2.4) among multiparas]. Odds ratios for preterm remained elevated after adjustment for covariates. Among …