Background Emerging evidence suggests that exposures in fetal life may have an impact many years later on the individual's susceptibility to diseases. Study of diet–disease associations over long time-spans is a major methodological challenge. We examined whether overall diet quality during pregnancy was associated with adolescent diet quality 15 years later in a prospective cohort representing a typical high-income population.

Methods Dietary intake assessed with a 360-item food frequency questionnaire during mid-pregnancy (1996–2003) was matched with offspring dietary intake assessed with a 150-item food frequency questionnaire at age 14 years in 19 620 mother–offspring pairs from the Danish National Birth Cohort (a large cohort of pregnant women with long-term follow-up of the offspring). Only children still in the cohort, living with mother, father, or both parents, and with valid parental personal identification number were invited to complete the food frequency questionnaire. A healthy eating index (HEI) was developed from Danish food-based dietary guidelines, and was based on eight components: fruits and vegetables, fish, dietary fibre, red meat, saturated fatty acids, sodium, sugar-sweetened beverages, and added sugar. HEI was divided into quartiles with those in the highest quartile having the more optimal diet. A log-linear binomial model was used to estimate the relative risk of the offspring being in the highest quartile of HEI at 14 years if the mother was also ranked in the highest quartile of HEI during pregnancy. Ethics approval was given by the National Committee on Health Research Ethics (H-4-2011-045). Written informed consent was obtained from the children's parents.

Findings At 14 years old, a child had an increased probability of being in the highest quartile of HEI if the mother was ranked in the highest quartile of HEI during pregnancy (relative risk 2·05, 95% CI 1·91–2·20). Adjustment for maternal pre-pregnancy body-mass index (BMI), parity, level of education, smoking, and energy intake of offspring did not affect the estimates, nor did analyses within strata of pre-pregnancy BMI, level of education, and maternal smoking.

Interpretation This is the first study, to our knowledge, to examine associations between maternal dietary habits during pregnancy and offspring dietary habits during adolescent years in a large contemporary cohort. Diet quality during pregnancy was associated with the quality of offspring diet at age 14 years. These findings could be driven by biological factors, social determinants, or both. Nevertheless, the findings have major potential implications for the developmental origins of health and disease since they emphasise the importance of taking both maternal and offspring diet into account when studying dietary causes of adult diseases.

Funding The study was supported by grants from the Danish Research Council, number 09-067124 (Centre for Fetal Programming) and number 09-075611; and by Nordea-fonden, number 02-2013-2014. The funders had no role in the design of the study, collection, analysis, interpretation of data, or writing of the abstract.