WEDNESDAY, July 3 (HealthDay News) -- Children born to first cousins or older mothers have a twofold higher risk of birth defects such as Down syndrome and heart and lung problems, but the absolute risk to any one child remains low, researchers say.
The findings from the study of more than 11,000 babies born in Bradford, England, between 2007 and 2011 are to be published online July 4 in The Lancet.
The absolute risk of a birth defect is small, rising from 3 percent in children born to unrelated parents to 6 percent for children born to cousins, the authors said. For mothers aged 35 or older, the risk rises to 4 percent, compared to 2 percent for women under 35.
This means that "only a small minority of babies born to couples who are blood relatives or older mothers will develop a [birth defect]," lead author Eamonn Sheridan, of the University of Leeds, said in a journal news release.
Bradford has a large Pakistani community. About 37 percent of Pakistani marriages in the study were between first cousins, compared with less than 1 percent of British marriages. The large number of marriages between first cousins accounted for 31 percent of birth defects for Pakistani babies.
The rate of birth defects among the babies born in Bradford was nearly double the U.K. rate, about 306 versus 166 per 10,000 live births. Low socioeconomic status did not explain the higher rate of birth defects among children born to first cousins, nor did mothers' smoking, drinking or obesity.
A high level of education among mothers halved the risk of having a baby with a birth defect, regardless of race/ethnicity, the researchers said.
Sheridan's team noted that marriage between blood relatives is common in many parts of the world. These communities need to be provided with clear information and counseling about the increased risk of birth defects among children born to blood relatives.
The U.S. Centers for Disease Control and Prevention has more about birth defects.