Parenthood is a unique turning point in women's careers. Mothers, but not fathers, experience large and persistent child penalties to earnings after the birth of their first child. Child penalties mainly result from three differences between mothers and fathers in response to childbearing: hours worked, labor market exit rates, and wages (Kleven, Landais, and Søgaard 2019). One possible explanation for these patterns is that the mental and physical strain of parenthood accrues more acutely to mothers, limiting their ability to perform on the job after having a child. Increased difficulty engaging in work may slow mothers' career advancement, driving longer-term gaps in earnings and labor force participation between mothers and fathers. In this paper, we explore whether physical ability to perform work tasks changes differently for mothers than for fathers after a first birth among service members in the U.S. Marine Corps. We estimate event study models around the first birth and include data on nonparents to estimate counterfactual trends. We find men's and women's physical performance drops after having a child. The effects for mothers are large and remain for at least two years, while for fathers, the declines are smaller and fade by their child's second birthday. Our findings provide a new angle on the child penalty literature, highlighting that changes to job performance for women in the immediate 24 months following a first birth could lead to long-term child penalties. [This paper was published in: "AEA Papers and Proceedings," Volume 112, 2022, pp. 561-67.]