Background: Omega 3 (n-3) and 6 (n-6) long-chain polyunsaturated fatty acids (LC-PUFAs) and the n-3:n-6 ratio are important for brain development. Whether maternal LC-PUFA status during pregnancy affects risk of problem behavior in later childhood is unclear. Methods: Within a population-based cohort, we measured maternal plasma docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and arachidonic acid (AA) concentrations and n-3:n-6-ratio in mid-pregnancy. Child emotional and behavioral problems at 6 y of age were assessed by parents (child behavior checklist), teachers (teacher report form), and combined parent/teacher report. results: Higher maternal DHA and n-3:n-6 ratio were associated with fewer child emotional problems using parent (odds ratio (OR) DHA = 0.82; 95% confidence interval (CI): 0.70, 0.96; P = 0.02 and OR n-3:n-6 = 0.83; 95% CI: 0.71, 0.96; P = 0.01; n = 5,307) and combined parent/teacher scores (OR DHA = 0.79; 95% CI: 0.66, 0.95; P = 0.01 and OR n-3:n-6 = 0.77; 95% CI: 0.65, 0.92; P < 0.01; n = 2,828). Higher AA was associated with more child behavioral problems using teacher (OR = 1.10; 95% CI: 1.00, 1.20; P = 0.04; n = 3,365) and combined parent/teacher scores (OR = 1.12; 95% CI: 1.02, 1.22; P = 0.02; n = 2,827). Maternal EPA was not associated with child problem behavior. conclusion: Indications of associations of maternal LC-PUFA status with child emotional and behavioral problems were found. Future research is needed to identify LC-PUFAsensitive periods of fetal brain development by including multiple assessments of prenatal LC-PUFA status. t wo families of polyunsaturated fatty acids (PUFAs), the omega 3 (n-3) and 6 (n-6) fatty acids (FAs), are nutrients that play a major role in brain development and functioning. They affect numerous processes, including membrane fluidity, neurotransmission, and gene expression (1). The long-chain PUFAs (LC-PUFAs), eicosapentaenoic acid (EPA; C20:5n-3), docosahexaenoic acid (DHA; C22:6n-3), and arachidonic acid (AA; C20:4n-6) are the main FAs that are metabolically active.In addition, the ratio between n-3 and n-6 FAs is important for optimal brain functioning. During the last century, however, the Western diet has provided a lower n-3 and higher n-6 intake than that in previous generations, diminishing the n-3:n-6 ratio (2).Because of their role in brain function and development, maternal LC-PUFAs during fetal development have been hypothesized to affect child problem behavior. Approximately half of the dry weight of the brain is lipid, of which 35% are PUFA, most of which is LC-PUFA (1). Increase in AA and DHA content of brain tissue starts during the third trimester of pregnancy (3), indicating that the fetal period possibly is a sensitive period. Moreover, the fetus mostly depends on maternal LC-PUFA supply for its development, because fetal LC-PUFA synthesis is limited (4).Research in this field is however scarce, and results are conflicting. Two studies on maternal fish intake, a major source of DHA, during pregnancy and offspring problem beha...