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Background: Maternal obesity during pregnancy is associated with an increased risk of obesity and metabolic disease in the offspring. Supplementation with fish oil (FO), which is insulin sensitizing, during pregnancy in mothers with overweight or obesity may prevent the development of greater adiposity and metabolic dysfunction in their children.

Objectives: To determine the effects of FO supplementation throughout the second half of pregnancy and lactation in mothers with overweight or obesity on infant body composition and metabolism.

Methods: A double-blind randomized controlled trial of 6 g FO (3.55 g/d of n-3 PUFAs) compared with olive oil (control) from mid-pregnancy until 3 mo postpartum. Eligible women had singleton pregnancies at 12-20 wk of gestation, and BMI >= 25 kg/m2. The primary outcome was the infant body fat percentage (DXA scans) at 2 wk of age. Secondary outcomes included maternal metabolic markers during pregnancy, infant anthropometry at 2 wk and 3 mo of age, and metabolic markers at 3 mo.

Results: A total of 129 mothers were randomized, and 98 infants had a DXA scan at 2 wk. Primary outcome: Imputed and nonimputed analyses showed no effects of FO supplementation on infant body fat percentage at age 2 wk. Secondary outcomes: There were no treatment effects on infant outcomes at 2 wk, but FO infants had a higher BMI z-score (P = 0.025) and ponderal index (P = 0.017) at age 3 mo. FO supplementation lowered maternal tri-glycerides by 17% at 30 wk of pregnancy (P = 0.0002) and infant triglycerides by 21% at 3 mo of age (P = 0.016) but did not affect maternal or infant insulin resistance. The rate of emergency cesarean section was lower with FO supplementation [aRR = 0.38 (95%CI 0.16, 0.90); P = 0.027].

Conclusions: FO supplementation of mothers with overweight or obesity during pregnancy did not impact infant body composition. There is a need to follow up the offspring to determine whether the observed metabolic effects persist. Clinical trial registry number: This study was registered with the Australian New Zealand Clinical Trials Registry (ACTRN12617001078347p). In addition, the Universal Trial Number, WHO, was obtained (U1111-1199-5860).