BackgroundWhen the vulnerabilities of the postnatal period are combined with the impact of the COVID-19 pandemic, psychosocial outcomes are likely to be affected. Specifically, we aim to: a) explore the psychosocial experiences of women in the early postnatal period; b) describe prevalence rates of clinically relevant maternal anxiety and depression; and c) explore whether psychosocial change occurring as a result of COVID-19 is predictive of clinically relevant maternal anxiety and depression. MethodsA sample of UK mothers (N = 614) with infants aged between birth and twelve weeks were recruited via convenience sampling. A cross-sectional survey design was utilised which comprised demographics, COVID-19 specific questions, and a battery of validated psychosocial measures, including the EPDS and STAI-S which were used to collect prevalence rates of clinically relevant depression and anxiety respectively. Data collection coincided with the UK government's initial mandated “lockdown” restrictions and the introduction of social distancing measures in 2020. FindingsDescriptive findings from the overall sample indicate that a high percentage of mothers self-reported psychological and social changes as a result of the introduction of social distancing measures. For women who reported the presence of psychosocial change, these changes were perceived negatively. Whilst seventy women (11.4%) reported a current clinical diagnosis of depression, two hundred and sixty-four women (43%) reported a score of ≥13 on the EPDS, indicating clinically relevant depression. Whilst one hundred and thirteen women (18.4%) reported a current clinical diagnosis of anxiety, three hundred and seventy-three women (61%) reported a score of ≥40 on STAI-S, indicating clinically relevant anxiety. After accounting for current clinical diagnoses of depression or anxiety, and demographic factors known to influence mental health, only perceived psychological change occurring as a result of the introduction of social distancing measures predicted unique variance in the risk of clinically relevant maternal depression (30%) and anxiety (33%). InterpretationTo our knowledge, this is the first national study to examine the psychosocial experiences of postnatal women during the COVID-19 pandemic in the UK. Prevalence rates of clinically relevant maternal depression and anxiety were extremely high when compared to both self-reported current diagnoses of depression and anxiety, and pre-pandemic prevalence studies. Perceived psychological changes occurring as a result of the introduction of social distancing measures predicted unique variance in the risk for clinically relevant maternal depression and anxiety. This study provides vital information for clinicians, funders, policy makers, and researchers to inform the immediate next steps in perinatal care, policy, and research during COVID-19 and future health crises.