Even though boys and girls in childhood perform similarly in math and spatial thinking, after puberty fewer young women pursue majors that emphasize abilities such as science, technology, engineering, and math (STEM) in college. If postpubertal feminization contributes to a lower likelihood of choosing STEM majors, then young women who enter puberty early should be the least likely to pursue those majors later in their education. In this study, we investigate the association between age at menarche and the choice of STEM major. We surveyed 150 undergraduate women from a variety of majors in a large, public university and created logistic regression models to estimate their likelihood of choosing a STEM major. We found that early-maturing girls are less likely to enter STEM majors. We posit that the earlier a young woman enters puberty, the earlier and more extensively she is affected by the "leaky pipeline."