AbstractBackgroundThe initial COVID‐19 pandemic response−related effects on conceptions following the use of assisted reproductive technologies (ART), and on changes in the maternal characteristics of women who conceived during the early vs. pre‐pandemic period, have been understudied.ObjectivesTo examine the effects of ART clinic closures in the United States (US) in March 2020 on the frequency of ART‐conceived live births, multiple births and stillbirths; and to describe changes in the characteristics of women who conceived in the early pandemic period.MethodsPopulation‐based cohort study including all births in the US from January 2015 to December 2020 (22,907,688 live births; 134,537 stillbirths). Interrupted time series (ITS) methodology was used to estimate rate ratios (RR) of expected versus observed rates in December 2020 (i.e., among births conceived mainly in March 2020). Demographic and clinical characteristics were compared between mothers who conceived in March 2020 versus March 2015–2019.ResultsOverall, 1.1% of live births and 1.7% of stillbirths were conceived by ART. ART‐conceived live births decreased by 57.0% in December 2020 (observed vs. expected RR 0.43, 95% confidence interval [CI] 0.40, 0.45), and these declines occurred in all subgroups of women. Multiple births also declined in December 2020. Stillbirth rates increased in December 2020 in ART‐conceived births (RR 2.55, 95% CI 1.63, 3.92) but remained unchanged in the non‐ART group. Maternal characteristics of women who conceived in the early pandemic versus pre‐pandemic period differed and included an increased prevalence of pre‐pregnancy obesity class 3 and chronic hypertension.ConclusionsThe early pandemic closure of ART clinics resulted in a substantial decline in ART‐conceived live births and multiple births in December 2020 and an increase in the proportion of stillbirths among ART‐conceived births. Women who conceived in the early pandemic period also had an increased prevalence of obesity and chronic hypertension.