My sister recently announced that she is pregnant.  Our family is overjoyed for her, and interested to see all developments.  Anyway, when she told me she went on to say, 'So no sushi for me.' This immediately got my sceptical brain humming, so I compiled what I could of the research I found on the internet and emailed her.  The content of the email is below, slightly edited to be a blog post.

After our conversation about pregnancy I did some research on the sushi question.  It turns out that there are three main factors: 1) problems with raw fish, 2) increased mercury levels that are unhealthy for a foetus, and 3) vitamin/dioxin issues. 

On the first problem, eating any raw fish can be a problem because of the possibility of parasites & funnies.  Parasites are worse when you are pregnant because they could affect the foetus, but there is no 'increased vulnerability' to parasites because you're pregnant. Note this does not rule out all sushi. For example, California rolls contain steamed crab which, because it is cooked, should contain no parasites.  Similarly for other cooked fish sushi, like crispy prawn rolls.  The crucial thing is to stay away from the uncooked eel, salmon, tuna, and cod.  The second thing with raw fish is that it (and soft cheeses like brie or camembert) has a higher likelihood than other foods to contain things called 'Listeria monocytogenes'.  But again, eating cooked sushi means
you avoid this.  Also, you could eat sushi if the fish has been frozen at  -20 degrees or lower (as this should kill most of the bad stuff).

On the second problem, the thing is to stay away from all uncooked and cooked swordfish, mackerel, shark, and tile fish (not like you regularly eat these anyway) because these have higher mercury levels than other fish.  High mercury levels, for which you'd have to eat a substantial amount of these fish, can damage a foetus's development or cause mutations.  If you don't eat these fish, then this shouldn't be a problem.

On the third issue, salmon can contain funny substances (dioxins) that can be harmful to foetuses in large  quantities, which means that you shouldn't eat salmon more than twice a week.  The second thing is that raw fish and shellfish contain substances (thiaminases) that break down vitamin B which babies need for growth. Again  not a problem if you're getting Vitamin B in supplements or in other foods, but it motivates not eating raw sushi more than twice a month, no problems for cooked sushi.    

All of this said, I couldn't find comprehensive studies actually showing that eating sushi is 'bad' for you during pregnancy (or at least worse for you than eating much of the crap that is in supermarkets these days). A lot of the hullabaloo seems to emanate from urban legends propagated in the US.  I'd probably consult a qualified
dietician (not a nutritionist) and ask them to give you a properly researched answer before you stop eating your favourite food for 9 months.  The most comprehensive online article I read suggests that as long as you don't eat raw sushi more than twice a month you should be fine.  Millions of Japanese people seem to do ok eating sushi while pregnant and there are lots of healthy Japanese babies as far as I know. Also, another article I found suggested that a US government study said that the risks of disease from eating cooked fish are about 1 in 2 million, whereas for cooked chicken it's about 1 in 25 000. So yes, on those Japanese and lots of fish...