Ask the expert: Is the mercury in fish likely to harm my unborn baby?
A researcher who's studied the effects of eating fish in pregnancy offers some reassurance. Lisa Salmon reports.
I've just found out I'm pregnant, so is it best to avoid fish like tuna because of its high mercury levels? I've heard mercury can cause autism in children.
Dr Caroline Taylor, a Wellcome Trust research fellow at the Centre for Child and Adolescent Health at University of Bristol, says: "We've been researching the effects of women eating fish in pregnancy, on the development of her baby before birth and into childhood. We measured mercury in the woman's blood during pregnancy and asked questions about how much fish she ate.
"The baby was followed up for several years, so we were able to measure many aspects of growth and development - including birth weight and prematurity, IQ, preschool development and behaviour.
"The studies have shown that eating fish in pregnancy doesn't increase the chances of poor development in the child, despite an increase in the mother's blood mercury level. Indeed, for some measurements, eating fish had a positively beneficial effect on the baby.
"Other studies in countries where women eat a lot more fish than in the UK have had similar findings. We think this is because fish is a rich source of other nutrients, such as iodine, omega-3 fatty acids, vitamin D and selenium, and these outweigh any 'bad' effects of mercury.
"Our most recent study has focused on diagnosed autism and autistic traits in the child. Again, we found that eating fish in pregnancy didn't make it more likely that the child would be diagnosed with autism, or have autistic traits. Indeed, for one of the traits, social cognition, the score was worse if the mother didn't eat fish in pregnancy.
"National guidance published by NHS Choices advises women to eat at least two portions of fish a week, one of which should be oily. For tuna, current advice is to eat no more than four medium-sized cans of tuna, or two fresh tuna steaks per week, so there's certainly no need to avoid tuna altogether (canned tuna counts as 'non-oily').
"Our research strongly endorses the advice to eat at least two portions of fish per week - this could be positively beneficial for your baby."