This study examines whether children with a younger sibling whose birth was unintended experience larger declines in the quality of their home environment and larger increases in behavioral problems than children whose younger sibling’s birth was intended. We use data from the NLSY79 to estimate cross-lag regression models that assess changes in the home environment and children’s behavioral problems after the birth of a sibling (intended or unintended). Results are consistent with our hypotheses, finding that, indeed, unintended births have negative spillover effects. Compared with children whose sibling’s birth was intended, both boys and girls whose sibling’s birth was unintended experienced larger declines in the quality of their home environment, and boys had larger increases in behavioral problems. We also find some unexpected evidence that mistimed births may have larger negative effects than unwanted births. This deserves further research, and we offer some possible explanations that could guide those investigations.