A growing body of literature demonstrates that prenatal maternal stress (PNMS) can influence infant and child outcomes across developmental domains. The timing of PNMS exposure may be particularly important, and late PNMS has predicted poorer emotion regulation outcomes in infancy and childhood. Behavioral indicators and measures of emotion regulation have differed widely in the existing PNMS literature, however. Additionally, despite the well-established use of the Still Face Procedure (SFP) to assess emotion regulation and infant-maternal interactions in the regulatory process, it has not been used within the context of PNMS. In the current research, the SFP was used in conjunction with a novel measurement of infant and maternal behaviors developed to assess infant emotion regulation in the context of maternal behavior and PNMS. A total of 100 infant-mother dyads were observed and coded during three, two-minute play episodes and two, two-minute Still Face episodes of the SFP via video recording. Both reinforcing (e.g., soothing/comforting) and non-reinforcing (e.g., punitive) maternal behaviors predicted numerous infant regulatory behaviors within the context of mostly early PNMS. In addition, late PNMS was found to differentially affect two regulatory behaviors based on infant sex.