ImportanceElevated maternal psychological distress during pregnancy is associated with altered fetal brain development. During the COVID-19 pandemic, prenatal maternal psychological distress more than doubled.ObjectiveTo examine the association of the pandemic and rising maternal psychological distress with brain growth in newborns using quantitative 3-dimensional volumetric magnetic resonance imaging (MRI).Design, Setting, and ParticipantsThis prospective cross-sectional study recruited mother-infant dyads at Children’s National Hospital, Washington, DC, during the COVID-19 pandemic (June 1, 2020, to June 30, 2022) into a longitudinal infant brain development study and compared them with an existing normative healthy cohort (recruited March 1, 2014, to December 31, 2019). Exclusion criteria included multiple gestation pregnancy, known or suspected congenital infection, documented chromosomal abnormalities, or any maternal contraindication to MRI, as well as prenatal COVID-19 exposure. Infants with structural brain abnormalities or a postnatal confirmation of a genetic syndrome were excluded.ExposurePsychological distress during COVID-19 pandemic.Main Outcomes and MeasuresPrenatal maternal mental health was evaluated using the Spielberger State-Trait Anxiety Inventory and the Perceived Stress Scale. Neonates underwent nonsedated brain MRI. An ordinary least squares linear regression model was used to measure the differences in regional brain volumes of neonates born before vs during the pandemic with and without exposure to elevated prenatal maternal psychological distress after adjustment for neonatal sex and gestational age at MRI and maternal age and educational level.ResultsA total of 159 mother-infant dyads were included in the analysis: 103 before and 56 during the pandemic (median gestational age of infants, 39.6 [IQR, 38.4-40.4] weeks; median maternal age, 34.5 [IQR, 31.0-37.0] years). Eighty-three infants (52.2%) were female. Among the mothers, 130 (81.8%) had a college degree and 87 (54.7%) had a graduate degree. Forty-four mothers (27.7%) identified as Asian, Hispanic, or multiracial; 27 (17.0%), as Black; and 88 (55.3%), as White. Scores on anxiety and stress measures were significantly increased in the pandemic cohort. Infants of mothers with elevated maternal distress showed median reductions in white matter (−0.36 [95% CI, −0.61 to −0.11] cm3; Q < .001), right hippocampal (−0.35 [95% CI, −0.65 to −0.06] cm3; Q = .04), and left amygdala (−0.49 [95% CI, −0.84 to −0.13] cm3; Q = .03) volumes compared with infants of mothers with low distress levels. After adjusting for the cohort effect of the pandemic, elevated trait anxiety remained significantly associated with decreased left amygdalar volumes (−0.71 [95% CI, −1.12 to −0.29]; Q < .001).Conclusions and RelevanceIn this cross-sectional study of maternal-infant dyads prior to and during the COVID-19 pandemic, regional neonatal brain volumes were associated with elevated maternal psychological distress.