Main findings
Although the findings made to date regarding the incidence of BDs associated with multiple and singleton pregnancies have been inconsistent, most studies have shown that multiple pregnancies are associated with a higher risk of BD than singletons.23, 24 However, univariate analysis alone was performed in the previous studies, and they did not categorize the BDs and had small sample sizes. In contrast, we studied multiple types of BD and the study had a large sample size of more than 1.9 million births, which should have facilitated the drawing of reliable conclusions. We found that multiple pregnancy is associated with a significantly higher risk of and a lower prenatal diagnosis rate for BDs than singleton pregnancy.