Mercury and Shellfish: Fish to Avoid during Pregnancy
A selection of Sashimi
Fish to Avoid during Pregnancy
You should not eat shell fish, raw fish or certain types of fish for a variety of reasons. The raw fish will contain a variety of bacteria which can cause food poisoning and during pregnancy your immune system is altered to focus on the baby and so the severity of your food poisoning is more likely to result in more serious symptoms. In a similar fashion to the way the salmonella infection poses a greater risk, you can read more about this at Salmonella and Pregnancy. So during this I will focus on the risk of anisakid worms from raw fish and mercury from other fish.
Anisakid worms and Raw Fish
One of the risks from eating raw shell fish and raw fish, in dishes such as sushi, is that they can contain parasites called anisakid worms, these can cause problems during pregnancy. Firstly they naturally leach nutrients which are needed by both you and your unborn baby. There is also a more serious consequence, where infection from the worm cause a condition called anisakidosis which causes symptoms such as:
severe abdominal pain
other gastrointestinal symptoms, including abnormal tissue growth (lesions) in the stomach or intestine
The disease is unpleasant and painful, and in very rare cases can be fatal. Eating fish contaminated with anisakid worms can also cause an allergic reaction.
This condition can cause serious problems for the baby and treatment should be sought immediately.
There is a simple way to avoid these problems, most of the sushi or raw fish products you buy from the supermarket has been frozen which is enough to kill the parasite, this means you can eat sushi and raw fish as long as it has been frozen. The FDA recommends all shellfish and fish intended for raw consumption be blast frozen to -35°C or below for fifteen hours or be regularly frozen to -20°C or below for seven days.
So to sum up, avoid all raw shellfish, cooked shellfish are fine. Avoid raw fish which has not been frozen, if it has been frozen to FDA approval it is fine to eat.
The Effects of Mercury on the Nervous System
Mercury in Fish
It is advised to avoid eating Shark, Swordfish and Marlin, you should limit the amount of tune you eat to:
no more than 2 tuna steaks a week (10g raw)
four cans of tuna week (140g drained)
The reason for this is that they often contain high levels of mercury, Methylmercury which is the most dangerous form of mercury, in fish mainly comes from mercury in ocean sediment that is transformed into methylmercury by micro organisms. This organic form of mercury is absorbed by the tissues of fish through their gills as they swim and through their digestive tracts as they feed.
Mercury levels differ from one species of fish to the next. This is due to factors such as the type of fish, size, location, habitat, diet and age. Fish that are predatory are large and at the top of the food chain, and so tend to contain more mercury due to the fact that not only do they get it from the environment but also in a more concentrated form from their food. The raise in the levels of Mercury in the mother means that it can pass across the Placenta to the foetus and so poses a serious risk to your baby It can damage your babies developing brain and nervous system. The fetus appears to be most sensitive to the effects of mercury during the third and fourth months of pregnancy. The effects on the brain and nervous system may not be noticed until developmental milestones - such as walking and talking - are delayed. Memory, language and attention span may also be affected.
Hope this clears up some confusion when it comes to fish and pregnancy, please check out some of the links below as they contain the latest research and some very interesting information.
- Mercury toxicity in the pregnant woman, fetus, and newborn infant. A review. - Abstract - UK PubMed
A research paper into the effect of Mercury on the unborn baby
- Abnormal neuronal migration, deranged cerebral cortical organization, and diffuse white matter... -
The effect of mercury on the development of foetal brains.