In a recent paper, Ichino and Moretti (2009) present evidence from a large Italian bank that much of the gap in absenteeism between women and men can be explained by absences with a 28-day cycle. These cyclical absences are interpreted as an effect of menstruation which can explain 14% of the gender earnings gap. While the health consequences of menstruation are undeniable, the general importance of menstruation in explaining gender gaps in absenteeism and earnings is unclear. In this paper, we show that 28-day cycles do not explain any of the gender gap in absences among teachers in the New York City public schools. Our results suggest that menstruation may not be an important determinant of absences for a large segment of the female labor force and that institutions greatly influence how biological gender differences affect labor market outcomes.