
Twins resulting from subfertility treatment are born about 4 days sooner (95% CI; 2.7?5.2) and are about 60% more likely (OR 1.6; 1.4?1.8) to be born slightly preterm (between 34 and 36 weeks) when compared with naturally conceived twins, according to the results of a population-based cohort study from Belgium. But the differences are largely explained by a first birth effect among subfertile couples and are mitigated by the protective effect of dizygotic twinning; most twins resulting from subfertility treatment are dizygotic.