Objective: To explore the effects of antenatal anxiety on fetal growth, and to investigate the effect of anxiety timing and the potentially different effect of trait anxiety and state anxiety on fetal growth. Design: Observational cohort study. Setting: Barcelona, Spain. Population: A cohort of 204 women with singleton pregnancies attending the antenatal clinic of a tertiary care setting during the strict lockdown of the COVID-19 pandemic in 2020. Methods: Psychosocial factors, maternal demographics, and obstetric outcomes were studied as potential predictors of low birth weight. The State-Trait Anxiety Inventory (STAIs, STAIt), the Edinburgh postpartum Depression Scale (EPDS) and the Medical Outcomes Study Social Support Survey (MOS-SSS) were used to assess symptoms of anxiety, symptoms of depression, and social support, respectively. Main Outcome Measures: Neonatal birth weight, head circumference and length. Results: There was a negative correlation between STAIt score (trait anxiety) and birth weight percentile (r=-0.228, p=0.047). In the univariate linear regression analysis, a lower maternal weight and BMI before pregnancy, parity, increased STAIt score and preterm birth below 37 weeks of gestation (p=0.008, p=0.015, p=0.028, p=0.047 and p=0.022, respectively) were identified as predictive risk factors for low birth weight. In the multivariate lineal regression analysis only a lower maternal weight before pregnancy and an increased STAIt score were independent predictors for low birth weight (p=0.020, p=0.049, respectively). Conclusions: Anxiety during pregnancy impacts birth weight, and specifically the trait anxiety, the one associated with the personality attributes, is a predictor for low birth weight.