This was published in the American Economic Review in 1994:
We estimate the long-run and life-cycle effects of unplanned children on unwed mothers by comparing unmarried women who first gave birth to twins and unmarried mothers who bore singletons. We find large short-term effects of unplanned births on labor-force participation, poverty, and welfare recipiency among unwed mothers, but not among married mothers. Although most of the adverse economic effects of unplanned motherhood dissipate over time for whites, there are larger and more persistent negative effects on black unwed mothers.
Notice that comparing one birth to two, rather than zero to one, tries to address the identification problem, namely that early pregnancy may be correlated with other unfavorable conditions. For the curious, here are many related articles. And here is a very useful literature review, which suggests inconclusive results.