Background: Maternal first cousins of an individual with a neural tube defect (NTD) are at increased risk for an NTD. It is not known if they are also at risk for other serious birth defects.
Methods: We carried out an interview study of uncles and aunts and first cousins in Irish NTD families covering their pregnancy histories and the health of family members.
Results: Maternal first cousins were more likely than paternal first cousins to have a birth defect (9.4% vs. 5.5%, p = 0.02; adjusted odds ratio: 1.72, 95% confidence interval: 1.04, 2.84).
Conclusions: This study shows that two generations of distant relatives (uncles/aunts and first cousins) in NTD families have similar maternal excesses of NTDs and birth defects overall. Inheritance mechanisms favouring matrilineal transmission, currently unknown, may contribute to birth defect occurrence in these families.