The descriptive epidemiology of conjoined twinning in the United States was 
investigated using data from the Birth Defects Monitoring Program (BDMP), a 
nationwide congenital malformations surveillance system that monitors discharge 
diagnoses associated with a third of the births in the United States. This is 
the largest recorded series concerning conjoined twins; data were analyzed on 
7,903,000 births monitored by the BDMP in the period 1970-1977. The analysis 
identified 81 sets of conjoined twins, for a crude incidence of 10.25 per 
million births. The most common types of conjoined twins were 
thoracoomphalopagus (28%), thoracopagus (18%), omphalopagus (10%), parasitic 
twins (10%), and craniopagus (6%). Conjoined twins are more common among females 
than males, and in nonwhites than whites. No maternal age effect was found. 
There was no evidence of seasonal or temporal clustering of the cases. The large 
number of conjoined twins who had birth defects that are not obviously linked to 
the conjoining (particularly neural tube defects and orofacial clefts) may 
provide insights into the pathogenesis of birth defects resulting from 
disordered embryonic migration and fusion.