Does fish consumption during pregnancy increase fetal growth? A study of the size of the newborn, placental weight and gestational age in relation to fish consumption during pregnancy

This study investigates whether consumption of fish during pregnancy may prolong gestation or increase fetal growth. From 1984 to 1987, 11 980 pregnant Danish women filled out a questionnaire while they were in the 36th week of gestation; this was 83% of all such women living in two geographically defined areas. The women were divided into four strata dependent on having consumed a fish meal 0, 1-2, 3-4, or 5+ times during the previous month. In nonsmokers, a one level increase in fish consumption was, after adjusting for 19 other variables in a multiple regression model, found to be associated with an increase of 11 g in placental weight, 0.08 cm in head circumference and 16 g in birthweight (95% confidence intervals (CI) 5 to 17 g (p=0.0002), 0.02 to 0.14 cm (p=0.02), and -2 to 34 g (p=0.09), respectively); no associations with birth length or gestational age could be detected (95% CI -0.07 to 0.11 cm (p=0.7) and -0.82 to 0.05 days (p=0.2), respectively, per level change in fish consumption). None of the associations seen in non-smokers were seen in smokers. We postulate that the associations seen in non-smokers could be due to marine n-3 fatty acids improving placental blood flow by increasing the ratio of prostacyclins to thromboxane. Smoking may possibly interfere with this mechanism via nicotine's inhibitory effect on platelet thromboxane production.