First cousins who have babies together have twice the risk of having a child with birth defects, according to a large UK study.
Children born to first cousins had a six percent chance of a birth abnormality, compared with an average of three percent.
The risk is comparable to babies born to mothers over the age of 35.
Marriage between cousins is legal in Australia under the Marriage Act 1961.
The researchers looked at almost 14,000 babies born between 2007 and 2011 in the UK city of Bradford, which has a large Pakistani population and first cousin marriage is culturally acceptable.
More than 2000 babies in the study were born to first-cousin parents and the authors found they were twice as likely to suffer a birth defect.
Nearly a third of birth defects in the Pakistani babies could be traced to marriage between relatives.
The researchers from the universities of Bradford and Leeds stressed that the increase in risk was small.
"At the heart of all this are children that are being born with often very distressing illnesses," Professor Neil Small, from the University of Bradford, told the UK's Independent.
"We customarily offer pre-conception and ante-natal advice that looks at areas like maternal age and health-related behaviours such as smoking and alcohol consumption. We think that in areas with high levels of consanguinity we could add to that health promotion package information about the risks associated with cousin marriage."
Professor Alan Bittles, from Murdoch University's Centre for Comparative Genomics, told ninemsn marriage between cousins is not common in Australia.
"We did a study in Western Australia about 10 years ago and found fewer than one percent of couples are cousins," he said.
"But of course, there has been fairly widespread migration into Australia from countries like North Africa and the Middle East and parts of South Asia, and these are parts of the world where 20 to 50 percent of marriages are between first cousins. So in recent migrant communities in Australia, you may find there is an increased rate of cousin marriage."
Professor Bittles said the overall risk was quite small.
"Sometimes when it's expressed as a doubling of the risk rate, in people's minds that sounds like a lot," he said.
"But when you say it's only an extra 3.6 percent, they realise it's not a lot."
Professor Bittles said it would be a good idea for marrying cousins to get genetic testing to confirm if they are carriers of certain conditions.
"If they think there might be a disease that seems a bit more common in our family than others, they could be tested to see if they are carriers," he said.
"It's important to note the authors are talking about congenital abnormalities and not all of them will have serious health consequences. The most common thing is a heart defect and in about 90 percent of cases they will spontaneously correct themselves in the first year of life."
The study was published in The Lancet.
Source: The Independent Author: Kimberly Gillan; Approving editor: Rory Kinsella