Why it's not a great idea to marry your cousin...

CHILDREN born to parents who are first cousins will have twice the risk of birth defects, roughly the same danger as those born to women older than 35.

In each case, the risk of defects increases from 1.5% to 3%.

The findings from the British study, and published in medical journal The Lancet, relied on data from 14,000 babies born between 2007 and 2011.

It found both cousin marriages and maternal age were associated with higher risk of these defects.

University of Auckland Associate Professor Andrew Shelling said children born from cousins were known to have a higher risk of defects, the study showed these were not particularly high.

"The increased rate of problems is relatively small, and in cousins, it is not much different to those babies born to unrelated individiuals," Prof Shelling said.

"In an interesting twist, and possibly a social comment, this study shows that the risk is about the same as older women - defined as having babies over 34 - having babies."

Marriage between first cousins are legal in Australia and New Zealand although it is outlawed in parts of the United States.