The prevalence of twinning and higher order multiple births is high and is increasing in many countries, due at least in part to fertility-enhancing medical therapies. About 1 in 40-45 births is a twin; it is thus important to investigate whether the twinning status confers a higher risk for morbidity and mortality, particularly early in life. Birth defects are a major cause of morbidity and mortality in the pediatric age-group. The purpose of this review is to consider the evidence for an

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... ased risk of some birth defects associated with twinning, using data from the literature and the Italian Multicentre Birth Defects Registry (IPIMC), to illustrate some of the challenging aspects of the study of birth defects in twins, and to suggest some guidelines for future investigations.Twins are a heterogeneous group. They may differ in origin and genetic similarity, as for monozygotic (MZ) and dizygotic (DZ) twins; and in placentation (monochorial monoamniotic, monochorial diamniotic, dichorial with separate or fused placentae). Likewise, birth defects are known to be heterogeneous in presentation, pathogenesis, and etiology. Thus, the question raised here is not whether or not birth defects 'in general' are more common in twins, but rather whether or not there are any specific types of defects that are more frequent among specific types of twins. To maintain the generality of the discussion, this review will focus on those structural defects which are not unique to twins; conjoined twins and arcadia, defects which are unique to the twinning process, will not be discussed.