Fish oil is commonly taken by pregnant women, and supplements sold at retail are often oxidised. We aimed to assess the effects of supplementation with oxidised fish oil during pregnancy in mothers and offspring, using a rat model, focussing on newborn viability and maternal insulin sensitivity. Female rats were allocated to a control or high-fat diet and then mated. They were subsequently randomised to receive a daily treatment of 1 ml of unoxidised fish oil, a highly oxidised fish oil, or control (water) throughout pregnancy by gavage. At birth, the gavage treatment was stopped, but the same maternal diets were fed ad libitum throughout lactation. Supplementation with oxidised fish oil during pregnancy had a marked adverse effect on newborn survival at day 2, leading to much greater odds of mortality than in the control (odds ratio 8.26) and unoxidised fish oil (odds ratio 13.70) groups. In addition, maternal intake of oxidised fish oil during pregnancy led to increased insulin resistance at the time of weaning (three weeks after exposure) compared to control dams (HOMA-IR 2.64 vs 1.42; p=0.044). These data show that the consumption of oxidised fish oil is harmful in rat pregnancy, with deleterious effects in both mothers and offspring.