This was not seen in pregnant rats given placebo (water) or unoxidised (fresh) fish oil.
"The findings have been interpreted differently by us as the researchers, the fish oil industry, the Government (Ministry of Primary Industries) and experts in the field leading to different messages to pregnant women," the two researchers say.
They felt it was time to "lay out the facts to empower consumers, particularly pregnant women, so that they may decide whether or not to take omega-3 fish oil capsules".
"Importantly this discussion relates to fish oil capsules and not fresh fish, the recommended way to achieve higher omega-3 levels."
Here, they list five facts about the study.
Fact 1: Omega-3 fish oil capsules taken by women during pregnancy has not been shown to improve intellectual outcome of their babies.
There are now several large studies in which mothers took omega-3 fish oil capsules from early pregnancy and there was no improvement in intellectual outcome in their offspring when assessed in early or mid-childhood.
Fact 2: Multiple studies by independent researchers from around the world have shown that fish oil capsules tested are frequently highly oxidised (to beyond recommended levels) at the time of purchase.
There are now five peer reviewed published studies conducted in five countries (our earlier study in NZ, USA, Canada, South Africa and most recently Finland) by research groups independent of the fish oil industry that all show the same pattern.
Most of the retail omega-3 fish oil capsules tested were oxidised usually to levels beyond that recommended.
Clearly there is an international issue with the quality of fish oils sold to consumers.
What this means is that in New Zealand, at the time of capsule purchase, it is not possible for a consumer to know whether fish oils are oxidised or not.
Fact 3: Highly oxidised fish oil given to rats throughout pregnancy led to a 30 per cent death rate in the rat babies by the second day of life.
In this study fish oil was oxidised in a controlled and consistent level so that all rat mothers were treated in the same way.
The fish oil was oxidised to twice the oxidation seen in the most highly oxidised products available for sale in New Zealand when we tested oxidation levels of all available omega-3 fish oil capsules.
We did this to ensure we would capture the full range of possibly oxidised fish oil available for sale.
Importantly we used a highly respected method to measure oxidation that is published in the European Pharmacopoeia.
This method is recommended by the Australian Therapeutic Goods Administration, the World Health Organisation and the Food and Agriculture Organisation of the United Nations.
Fact 4: A study in which pregnant women are treated with oxidised omega-3 fish oil capsules to determine whether there are harmful effects on mother or baby will never be conducted.
No such study has ever been conducted and nor will it.
It would be completely unethical to conduct a study in humans in which the underlying reason to do the study is to determine whether oxidised fish oil is harmful to mother and baby.
What we can learn about the effects of oxidised fish oil in pregnancy will be limited to animal studies.
Inevitably this will leave women having to make a decision about whether to take fish oil capsules in pregnancy based upon animal studies which are not directly translatable to humans.
Fact 5: Omega-3 fish oil manufacturers in New Zealand do not perform independent oxidation testing of each batch of capsules and publicise these results on the bottle.
This simple measure would inform consumers who wish to buy omega-3 fish oils that the product is not oxidised.
This is a reasonable consumer expectation.
This could be achieved either voluntarily by manufacturers or alternatively imposed by Government (the Ministry of Primary Industry).