Breastfeeding may benefit child cognitive development, but few
studies have quantified breastfeeding duration or exclusivity, nor has
any study to date examined the role of maternal diet during lactation on
child cognition.

To examine relationships of breastfeeding duration and
exclusivity with child cognition at ages 3 and 7 years and to evaluate
the extent to which maternal fish intake during lactation modifies
associations of infant feeding with later cognition.

Design, Setting, and Participants 
Prospective cohort study (Project Viva), a US prebirth cohort
that enrolled mothers from April 22, 1999, to July 31, 2002, and
followed up children to age 7 years, including 1312 Project Viva mothers
and children.

Main Exposure 
Duration of any breastfeeding to age 12 months.

Main Outcomes and Measures 
Child receptive language assessed with the Peabody Picture
Vocabulary Test at age 3 years, Wide Range Assessment of Visual Motor
Abilities at ages 3 and 7 years, and Kaufman Brief Intelligence Test and
Wide Range Assessment of Memory and Learning at age 7 years.

Adjusting for sociodemographics, maternal intelligence, and
home environment in linear regression, longer breastfeeding duration was
associated with higher Peabody Picture Vocabulary Test score at age 3
(0.21; 95% CI, 0.03-0.38 points per month breastfed) and with
higher intelligence on the Kaufman Brief Intelligence Test at age 7
(0.35; 0.16-0.53 verbal points per month breastfed; and 0.29;
0.05-0.54 nonverbal points per month breastfed). Breastfeeding duration
was not associated with Wide Range Assessment of Memory and Learning
Beneficial effects of breastfeeding on the Wide Range Assessment
of Visual Motor Abilities at age 3 years seemed greater for women who
consumed 2 or more servings of fish per week (0.24; 0.00-0.47 points per
month breastfed) compared with less than 2 servings of fish per week
(−0.01; −0.22 to 0.20 points per month breastfed) (P = .16 for interaction).

Conclusions and Relevance 
Our results support a causal relationship of breastfeeding
duration with receptive language and verbal and nonverbal intelligence
later in life.