Background: Fetal life and infancy might be critical periods for brain development leading to increased risks of neurocognitive disorders and psychopathology later in life. We examined the associations of fetal and infant weight growth patterns and birth characteristics with behavior and cognitive outcomes at the age of 13 years. Methods: Population-based prospective cohort study from fetal life until adolescence. Pregnant women with a delivery date between April 2002 and January 2006 were eligible. Follow-up measurements were available for 4716 children. Fetal weight was estimated in the second and third trimesters of pregnancy by ultrasonography. Infant weight was measured at birth and at 6, 12, and 24 months. Fetal and infant weight acceleration or deceleration were defined as a change in SD greater than 0.67 between time points. Total, internalizing and externalizing problems and attention-deficit hyperactivity disorder (ADHD) symptoms were measured using Child Behavior Checklist (CBCL/6-18), autistic traits by the Social Responsiveness Scale (SRS) and intelligence quotient (IQ) by the Wechsler Intelligence Scale for Children-Fifth Edition (WISC-V). Results: One week longer gestational age at birth was associated with a -0.03 SDS (95% Confidence Interval (CI): -0.04, -0.01) lower total behavior problems score, a -0.02 SDS (95% CI: -0.04, -0.01) lower ADHD symptoms score. Also an increase in birth weight of 500 g was associated with a lower odds of having high externalizing problems (OR 0.92 (95% CI: 0.86, 0.98) and of having a low IQ score (OR 0.79 (95% CI: 0.71, 0.88). Compared to children with normal fetal and infant growth, those with accelerated fetal and infant growth had a 0.27 SDS higher IQ (95% Confidence Interval 0.11, 0.44). Conclusions: Both fetal and infant weight development are associated with behavioral and cognitive outcomes in early adolescence. Follow-up studies are needed to assess whether these associations link to later life mental health outcomes.