Background/objectives:Studies suggest that intake of marine n-3 polyunsaturated fatty acids (n-3 PUFA) in pregnancy have an impact on birth weight, but only few have investigated the effect on early fetal growth. The objective of the study was to investigate the association between levels of PUFA in maternal blood in gestational week 24 and biometric measures and estimated fetal weight in gestational week 20.Subjects/methods:In the COPSAC cohort, whole-blood fatty acid composition (a biomarker of PUFA intake) from 583 women in week 24 was analyzed by gas chromatography. Biometric data (head circumference, abdominal circumference and femur length) were collected by ultra sound in week 20 and fetal weight was estimated. Associations between whole-blood PUFA (docosahexaenoic acid (DHA), total n-3 PUFA, n-6/n-3 PUFA, total n-6 PUFA) and fetal weight and biometrics measures were analyzed by multivariable-adjusted linear regression analyses. Results:There was a wide range in maternal blood DHA, which varied from 1.8 to 6.9% depending on socioeconomic status, smoking and body mass index. After adjusting for these variables, no association was observed between any of the assessed PUFA components and the circumference of head or abdomen or fetal weight. However, an inverse association was established between DHA and total n-3 PUFA and femur length (P