Among the 25 446 mother-child pairs with information on 18-mo development, mean maternal fish intake was 26.6 (range: 0–493.9) g/d. Maternal fish intake increased from a mean of 5.4 (range: 0–10.5) g/d in the lowest quintile of intake to 22.3 (range: 18.2–26.8) g/d in the middle quintile and 58.6 (range: 39.4–493.9) g/d in the highest quintile. Thus, on average, women in the lowest quintile consumed <1 fish serving/wk, those in the middle quintile consumed ≈1.5 fish servings/wk, and those in the highest quintile consumed ≈3.5 fish servings/wk. Most (86.3%) of the women reported consuming 1–2 fish servings/wk (1–340 g/wk), and 11.0% consumed ≥3 fish servings/wk (>340 g/wk), whereas only 2.8% of women never consumed fish (0 g/wk).
The fish species most frequently consumed by women in the DNBC were cod, plaice, salmon, herring, and mackerel. The median mercury content of these species, according to the Danish food monitoring program, ranges from 0.034 to 0.049 ppm (26
). These fish types account for ≈85% of the total seafood intake in the study population. Species with high mercury content, such as shark and king mackerel, are not commonly consumed in Denmark.
Compared with mothers in the lowest quintile of prenatal fish intake, those in the highest quintile were slightly older, more likely to be of high socioeconomic status, more likely to be high school graduates, and more likely to use alcohol but less likely to be nulliparous or unmarried and to smoke cigarettes during pregnancy (). The mean duration of any breastfeeding was 7.9 mo. Mothers with higher fish intake had infants who were heavier at birth and who tended to breastfeed for a longer duration.
Child motor, social or cognitive, and total development scores also varied by maternal fish intake (). For example, 5.7% of children with mothers in the lowest quintile of fish intake had the lowest total development score at 18 mo, whereas only 3.5% of children with mothers in the highest quintile of fish intake had the lowest total development score ().
In general, participant characteristics were associated with child development in the anticipated directions (). Thus, predictors of a higher child developmental score at 18 mo included having a mother who was younger and (previously) nulliparous or who did not drink alcohol during pregnancy, having a father with higher education, being female, and being born after longer gestation or with higher birth weight (). One association went in the direction opposite that anticipated (namely, that children of mothers who smoked cigarettes had higher developmental scores); this finding was mainly based on higher motor development (data not shown), which was perhaps due to the fact that children born to smoking mothers are more likely to be hyperactive (27
Associations of parent and child characteristics with child total developmental score at 18 mo among 25 446 children in the Danish National Birth Cohort
In the unadjusted analysis, higher maternal fish intake was associated with a higher total developmental score at age 18 mo (unadjusted OR: 1.28; 95% CI: 1.19, 1.36 for the highest compared with the lowest quintile of fish intake). Adjustment for parent and child characteristics did not markedly influence estimates. For example, the effect of maternal fish intake in the highest compared with the lowest quintile was 1.27 (95% CI: 1.18, 1.36) after adjustment for child characteristics and 1.30 (95% CI: 1.21, 1.40) after additional adjustment for parental characteristics. Further adjustment for other potential maternal dietary predictors of child development, namely iron and folic acid intakes, did not alter estimates for fish consumption (data not shown).
After additional adjustment for breastfeeding duration, higher maternal prenatal fish intake remained associated with higher child developmental scores at 18 mo, with an OR of 1.29 (95% CI: 1.20, 1.38) for the highest compared with the lowest quintile (). When we instead analyzed fish intake according to current guidelines for intake during pregnancy, as was done previously (12
), ORs for higher total development at 18 mo were 0.98 (95% CI: 0.85, 1.12) for 1–340 g/wk and 1.20 (1.04, 1.40) for >340 g/wk, compared with 0 g/wk. When we expressed fish intake as a continuous exposure, the OR for higher development was 1.49 (95% CI: 1.33, 1.66) for each additional fish serving/wk. Estimates of the associations of prenatal fish intake with motor and social or cognitive development were generally similar, and approximately the same for milestones reported at 6 mo as for those reported at 18 mo (OR for higher total development at 6 mo: 1.25; 95% CI: 1.17, 1.34 for highest compared with lowest quintile of fish intake) ().
Associations of maternal prenatal fish intake [by quintile (Q)] and duration of infant breastfeeding with attainment of developmental milestones at ages 6 mo (n = 28 958) and 18 mo (n = 25 446) among children in the Danish National Birth Cohort
The adjusted ORs for the association of maternal prenatal fish intake (in quintiles) with attainment of a greater number of developmental milestones at age 18 mo are shown in . Estimates were similar for the lowest and second quintile and then increased across the 3 highest quintiles of intake ().
FIGURE 1 Associations of maternal prenatal fish intake, in quintiles, with total developmental milestones attained by children at 18 mo of age. n = 25 446 children in the Danish National Birth Cohort. Odds ratios from cumulative ordinal logistic regression analysis (more ...)
Longer duration of breastfeeding was also associated with better development at age 18 mo. The effect estimate was minimally changed after adjustment for parent and child characteristics; the unadjusted OR was 1.33 (95% CI: 1.23, 1.43); after adjustment for parent and child characteristics and for breast-feeding duration of ≥10 mo compared with ≤1 mo, the OR was 1.29 (95% CI: 1.19, 1.40). After additional adjustment for fish intake, longer breastfeeding duration remained associated with a greater number of attained developmental milestones (OR: 1.28; 95% CI: 1.18, 1.38 for breastfeeding ≥10 mo compared with ≤1 mo). The effect of breastfeeding on the social or cognitive outcomes appeared similar to that on the motor outcomes (). Associations were somewhat weaker for outcomes assessed at 6 mo than for those assessed at 18 mo ().
Because we were concerned that we were not adequately accounting for the effects of differences in maternal characteristics according to fish intake, we next restricted our analyses to the children of mothers who had normal prepregnancy BMI and high socioeconomic status, who were nonsmokers and married, and who had no postpartum depression. In this subset of 11 274 children with 18-mo outcome data, the OR was 1.26 (95% CI: 1.12, 1.40) for the highest compared with the lowest quintile of maternal prenatal fish intake, and 1.22 (95% CI: 1.05, 1.41) for breastfeeding ≥10 mo compared with ≤1 mo.
Among women who breastfed for >6 mo (61.4% of the population), the association of prenatal fish intake with their infant’s total developmental score at 18 mo (OR: 1.34; 95% CI: 1.23, 1.48 for the highest compared with the lowest quintile) was, if anything, slightly stronger than the effect estimate among those who breastfed up to 6 mo (OR: 1.23; 95% CI: 1.10, 1.38). However, the interaction term for breastfeeding and fish intake did not suggest any effect modification (P = 0.77).