Bennett et al, Genetic Counseling and Screening of Consanguineous Couples and Their Offspring: Recommendations of the National Society of Genetic Counselors

"Few studies document the actual risks to the offspring of consanguineous unions. The risks quoted for birth defects and mental retardation are often based on studies of non-Western populations where consanguineous unions are common, and they may not be applicable to consanguineous unions in the United States and Canada (Al-Abdulkareem and Balal, 1998; Al-Awadi et al., 1985; Al-Gazali et al., 1997; Bittles, 1998, 2001; Bittles et al., 1991; Bittles and Neel, 1994; Harper, 1998; Jaber et al., 1997, 1998; Kaku and Freire-Maia, 1992; Madhaven and Narayan, 1991; Schull and Neel, 1965; Shami et al., 1991; Vogel and Motulsky, 1996). Furthermore, in all such studies, the criteria for what is considered a significant medical problem in offspring are not standardized. Studies using excess mortality to measure the adverse effects of inbreeding often did not account for the effects of sociodemographic variables such as maternal age, birth interval, socioeconomic status, and maternal education, thereby exaggerating the adverse effect of consanguinity (Bittles, 1998; Kaku and Freire-Maia, 1992). The risk of adverse medical outcomes for the offspring of consanguineous unions, as compared to a baseline risk for the general population, is reviewed under Baseline Risk for the Offspring of Consanguineous Unions Compared to Those From Nonconsanguineous Unions.

... Given the almost universal cross-cultural stigma, social disapproval, and legal sanctions to incestuous unions, there is a paucity of data regarding adverse medical outcomes in the offspring of incestuous unions. Published studies are fraught with significant ascertainment biases. These biases, such as lack of paternity documentation, young maternal age, possible parental disease and/or intellectual impairment, parental socioeconomic status (or lack of report of this variable), and complications of unsuccessful attempted pregnancy termination (Bittles, in press). Table IV summarizes the four most comprehensive published studies of incest (Adams and Neel, 1967; Baird and McGillivray, 1982; Carter, 1967; Seemanova, 1971). Three of these studies were retrospective, and the controls for matched nonincestuous pregnancies were highly variable (Baird and McGillivray, 1982; Carter, 1967; Seemanova, 1971). These studies are also limited in the number of years that the incestuous progeny were followed. Although the highest risk for morbidity and mortality would be expected in the first year of life, moderate medical problems and mental retardation would not be evident until later."