Regions of homozygosity (ROH) increase the risk of recessive disorders, and guidelines recommend reporting of excessive ROH in prenatal testing. However, ROH are common in populations that practice endogamy or consanguinity, and cutoffs for reporting ROH in such populations may not be evidence-based. We reviewed prenatal testing results (based on cytogenetic microarrays) from 2191 pregnancies in the Jewish and non-Jewish populations of Northern Israel and estimated the prevalence of ROH according to self-reported ethnicity and parental relationships. The proportion of the genome in ROH, ROH rate, was higher in non-Jews [Mean (SD) = 2.91% (3.92%); max = 25.54%; N = 689] than in Jews [Mean (SD) = 0.81% (0.49%); max = 3.93%; N = 1502]. In the non-Jewish populations, consanguineous marriages had the highest ROH rates [Mean (SD) = 7.14% (4.55%), N = 217], followed by endogamous [Mean (SD) = 1.13% (1.09%), N = 283] and non-endogamous [Mean (SD) = 0.69%(0. 56%), N = 189] marriages. ROH rates were greater than 5%, the ACMG-recommended cutoff, in 149/689 (21.63%) of the non-Jewish samples. Within the Jewish populations, the rates were similar between Ashkenazi, North African, and Middle Eastern Jews, but were higher for six consanguineous unions [Mean (SD) = 2.38% (1.23%)] and when spouses belonged to the same sub-population. Given the high ROH rates we observed in some subjects, we suggest that assessing the risk for recessive conditions in consanguineous/endogamous populations should be done before the first pregnancy, through genetic counseling and sequencing. Such an approach will: (1) identify couples who are at risk and counsel them on reproductive options; and (2) avoid the stress that couples who are not at risk may experience due to a prenatal ROH report.
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