Conjoined twins (CT) are a very rare developmental accident of uncertain etiology. Prevalence has been previously estimated to be 1 in 50,000 to 1 in 100,000 births. The process by which monozygotic twins do not fully separate but form CT is not well understood. The purpose of the present study was to analyze diverse epidemiological aspects of CT, including the different variables listed in the Introduction Section of this issue of the Journal. The study was made possible using the International Clearinghouse for Birth Defects Surveillance and Research (ICBDSR) structure. This multicenter worldwide research includes the largest sample of CT ever studied. A total of 383 carefully reviewed sets of CT obtained from 26,138,837 births reported by 21 Clearinghouse Surveillance Programs (SP) were included in the analysis. Total prevalence was 1.47 per 100,000 births (95% CI: 1.32-1.62). Salient findings including an evident variation in prevalence among SPs: a marked variation in the type of pregnancy outcome, a similarity in the proportion of CT types among programs: a significant female predominance in CT: particularly of the thoracopagus type and a significant male predominance in parapagus and parasitic types: significant differences in prevalence by ethnicity and an apparent increasing prevalence trend in South American countries. No genetic, environmental or demographic significant associated factors were identified. Further work in epidemiology and molecular research is necessary to understand the etiology and pathogenesis involved in the development of this fascinating phenomenon of nature.

Our objective was to evaluate the frequency and type of malformations associated with gastroschisis in a large pool of international data, to identify malformation patterns, and to evaluate the role of maternal age in non-isolated cases. Case-by-case information from 24 registries, all members of the International Clearinghouse for Birth Defects Surveillance and Research (ICBDSR), were evaluated. After the exclusion of other abdominal wall defects cases were classified as: (a) isolated; (b) recognizable syndrome, chromosomal or not; (c) multiple congenital anomalies (MCA). Our results showed that out of 3,322 total cases 469 non-isolated cases were registered (14.1%): 41 chromosomal syndromes, 24 other syndromes, and 404 MCA. Among MCA four groups of anomalies were most frequent: CNS (4.5%), cardio-vascular (2.5%), limb (2.2%), and kidney anomalies (1.9%). No similar patterns emerged except two patterns resembling limb-body wall complex and OEIS. In both of them the gastroschisis could be however misclassified. Chromosomal trisomies and possibly non-syndromic MCA are associated with an older maternal age more than isolated cases. On consideration of our data and the most valid studies published in the literature, the best estimate of the proportion of gastroschisis associated with major unrelated defects is about 10%, with a few cases associated to recognizable syndromes. Recognized syndromes with gastroschisis seem to be so exceptional that the well documented and validated cases are worth being published as interesting case report. An appropriate case definition in etiological studies should include only isolated gastroschisis after an appropriate definition of isolated and non-isolated cases and a thorough case-by-case review.

OBJECTIVE To identify trends in the prevalence and epidemiologic correlates of gastroschisis using a large population-based sample with cases identified by the National Birth Defects Prevention Network over the course of an 11-year period. METHODS This study examined 4,713 cases of gastroschisis occurring in 15 states during 1995–2005, using public use natality data sets for denominators. Multivariable Poisson regression was used to identify statistically significant risk factors, and Joinpoint regression analyses were conducted to assess temporal trends in gastroschisis prevalence by maternal age and race and ethnicity. RESULTS Results show an increasing temporal trend for gastroschisis (from 2.32 per 10,000 to 4.42 per 10,000 live births). Increasing prevalence of gastroschisis has occurred primarily among younger mothers (11.45 per 10,000 live births among mothers younger than age 20 years compared with 5.35 per 10,000 among women aged 20 to 24 years). In the multivariable analysis, using non-Hispanic whites as the referent group, non-Hispanic black women had the lowest risk of having a gastroschisis-affected pregnancy (prevalence ratio 0.42, 95% confidence interval [CI] 0.37–0.48), followed by Hispanics (prevalence ratio 0.86, 95% CI 0.81–0.92). Gastroschisis prevalence did not differ by newborn sex. CONCLUSIONS Our findings demonstrate that the prevalence of gastroschisis has been increasing since 1995 among 15 states in the United States, and that higher rates of gastroschisis are associated with non-Hispanic white maternal race and ethnicity, and maternal age younger than 25 years (particularly younger than 20 years of age).