Association of twinning and maternal age with major structural birth defects in Utah, 1999 to 2008.

Background: Twinning is a risk factor for birth defects. Using population-based data from the Utah Birth Defect Network and the Utah Department of Health Vital Records, we investigated whether twinning increases the risk of major structural birth defects in live-born opposite-sex (OS) and same-sex (SS) twins as proxies for monozygotic and dizygotic twins, respectively. We also assessed whether maternal age modified the association between twinning and birth defects.