Dr Ben Albert (pictured) led the research with Professor Wayne Cutfieldat the University of Auckland-based Liggins Institute.
Fresh fish oil could one day be used to prevent babies from becoming overweight and developing diabetes later in life.
An Auckland University study has found the oil, given to overweight pregnant rats, reduced the risk of their offspring developing the disease.
Fresh fish oil reduced the risk of baby rats developing diabetes.
But for now, researcher Dr Ben Albert cautions against pregnant women taking fish oil supplements on the basis of his finding.
"At this stage we do not recommend that pregnant women take fish oil for their babies' health for two reasons," Albert said.
"First, our findings need confirmation in human studies. Second, you can't know if the supplements you buy are fresh or not, and we don't yet know the safe level of oxidisation for fish oil in pregnancy."
In the first of his team's fish oil studies, published in 2015, the researchers tested 36 brands of fish oil supplements capsules.
Most of them were oxidised - on their way to becoming rancid - beyond international recommended levels. The deterioration had nothing to do with best-before date, price, or the country they came from.
"Our advice would be, if you want more omega 3 fats, eat oily fish," Albert said.
In the latest experiment, pregnant rats were fed either a high-fat diet to make them overweight, or a standard diet, and half from each group were also given fresh fish oil. In the offspring of the high-fat group, the fish oil treatment improved the way insulin works, which is protective against diabetes.
Later this year, Albert will lead a clinical trial of fish oil supplements in overweight pregnant women to see if the supplements can indeed reduce the risk of their babies becoming overweight and of developing diabetes in later life.
His team has received funding for the first phase from the Health Research Council, National Science Challenge A Better Start, and Cure Kids.
Children of overweight women have nearly twice the risk of developing problems like diabetes later in life, and at least three in 10 pregnant women in New Zealand are overweight.
Up to one in five New Zealand women currently take fish oil supplements during pregnancy, according to latest estimates.