Following reports between 2010 and 2015 of clusters of cases involving children with transverse agenesis of the upper limbs (TAUL) in three departments—Ain, Loire-Atlantique, and Morbihan, the Ministries of Solidarity and Health, Ecological and Solidarity Transition, and Agriculture and Food referred the matter to Santé publique France and ANSES. This referral requested that the agencies conduct further investigations into these clusters of cases (also known as aggregates) and carry out a literature review aimed at identifying and prioritizing risk factors for ATMS, particularly environmental ones. This work was to be conducted as part of a collective expert assessment. The Scientific Expert Committee (CES), established for this purpose to ensure the independence and transparency of the assessment, published an initial report in July 2019. In this report, it recommended, in particular, conducting additional investigations into the confirmed cluster in a municipality in Morbihan to better characterize the exposures during pregnancy of the mothers of the affected children. The CES also recommended a re-analysis of the suspected cluster in a commune in Loire-Atlantique, once the processes of identifying potential ATMS cases in that department and validating the diagnosis by genetic experts had been completed. This supplementary report therefore follows up on the one published in July 2019. In it, the CES also comments on the relevance of continuing the planned literature review. To issue its expert opinion, the CES relied on the supplementary work of the two health agencies within the scope of their respective mandates. This supplementary CES report on ATMS highlights the difficulties in identifying a common exposure in clusters of health events as rare as ATMS. It also highlights the limitations associated with the case identification process when this research is conducted several years after the children’s birth. The CES reiterates that it is now necessary to strengthen the system for monitoring congenital anomalies in France (creation of a seventh registry for monitoring congenital anomalies, establishment of a national multi-source surveillance system) and to improve understanding of their risk factors in order to implement targeted prevention measures.