Objective: To examine the effect of prenatal maternal anxiety on birthweight and preterm birth, controlling for genetic confounding using a sibling comparison design. Design: This is a population-based prospective cohort study with a comparison of a population level analysis and a sibling analysis. Setting: This study is based on the Norwegian Mother, Father and Child Cohort Study (MoBa) conducted by the Norwegian Institute of Public Health (https://www.fhi.no/en/studies/moba/ ). Sample: Women and their chiold participating in the MoBa (n= 78,117) and women participating with more than one pregnancy (n=12,480). Methods: Associations between prenatal maternal anxiety (measured across the 17th and 30th weeks) and birth outcomes (birthweight and gestational age) were examined using linear regression with adjustment for family-shared confounding in a sibling comparison design. Main outcomes: Birthweight (in grams) and gestational age (ultrasound measure in days) were obtained from the Medical Birth Registry of Norway. Results: The maternal anxiety score during pregnancy was inversely associated with newborn’s birthweight (Beta = -112,8 95% CI: –142.7, -83.0) and gestational age (Beta=-1.77, 95% CI: -2.42, -1.13) after adjustment for several covariates. The association of the maternal anxiety score with both newborn’s birthweight (Beta=-173.9, 95% CI: -252.3, -95.4) and gestational age (Beta=-1.08, 95% CI: -2.91, -0.75) remained but was largely weakened after further adjusting for the shared-family confounding in the sibling comparison design. Conclusion: The link between maternal prenatal anxiety and birthweight and gestational age remained after adjusting for shared family confounding, yet estimates were weakened after adjusting for environmental covariates.