Is there a gender gap in the serial founding of VC-backed startups? Despite robust evidence linking serial entrepreneurship to startup success, women comprise only 13.3% of VC-backed founders, declining to just 4% among those who found three or more startups. We introduce a novel empirical design that exploits within-startup variation to study this gap, comparing future funding outcomes for men and women who co-founded the same startup. This approach, akin to twin studies, controls for unobservable differences in startup quality and founder ability. We document substantial gender gaps, both on average and following failure or success of the current startup. Following failure, women are 22.5% less likely to found another VC-backed startup compared to their cofounders who are men. Among those who do found another VC-backed firm, women raise 53.3% less capital following failure and 24.6% less following success. Using founder LinkedIn data and pension fund supply shocks, we investigate potential demand- and supply-side drivers of these gaps. We rule out lack of interest by women in founding new firms and find no evidence of gender differences in founder quality. In fact, subsequent startups founded by women and men have higher success probabilities, despite the large funding gap. The gender gaps appear driven by unequal treatment, particularly from new investors rather than existing ones. Our analysis reveals striking negative spillovers following investors' experiences with other women-founded startup failures, but no positive spillovers following their successes, consistent with theories of stereotyping. These findings suggest potential efficiency gains from reducing frictions faced by experienced women founders in accessing venture capital.