Mercury levels in canned tuna are 'unpredictable' and pregnant women should avoid eating them to be safe, according to a report.

An investigation by Consumer Reports tested 30 cans of skipjack and albacore tuna from five popular brands, including Bumble Bee, StarKist and Chicken of the Sea.

Every single one contained mercury, but there were wide fluctuations in levels between cans — even those in the same brand. Three cans contained so much mercury that researchers said they should not be eaten at all.