Statistical analyses
Maternal age is presented as mean and standard deviation (SD). Categorical data are presented as number (N) and percentage. The incidence of BD was calculated as the number of BDs per 1,000 births. Differences in the incidence of BD and the characteristics of the BDs associated with singleton and multiple pregnancies were analyzed using ANOVA with respect to maternal age and the chi-square test for categorical variables. The chi-square test was also used to identify differences in the prenatal diagnosis rates for BDs associated with singleton and multiple pregnancies.
Multivariate logistic regression models were constructed to evaluate the relationships between the prenatal diagnosis of BDs with multiple pregnancies, after adjustment for the year of birth, maternal age group, and maternal domicile. For the BDs that were diagnosed prenatally, we further categorized the time of diagnosis as <28 or ≥28 weeks of gestation, then chi-square testing was used to identify differences in the timing of diagnosis between singleton and multiple pregnancies, considering BDs in their entirety and specific types of BD that were found to significantly differ in their prenatal diagnosis rate.
All analyses were carried out using STATA 13 (StataCorp, College Station, TX, United States).