Recent increases in births to unmarried parents, and the instability surrounding these relationships, have raised concerns about the possible health effects associated with changes in family structure. Using data from the Fragile Families and Child Well-Being Study (N = 2,448), this article examines trajectories of maternal mental and physical health. We specifically focus on mothers' transitions into and out of residential relationships with a child's biological father during the first five years after birth. We find that continuously married mothers are in better mental and physical health than unmarried mothers one year after birth, but the disparity does not increase over time. This finding provides little support for the resource model. Consistent with the crisis model, exiting a marital or cohabiting union increases mental health problems and decreases self-rated health. These effects appear to be relatively short-lived, though, and they are stronger for mental health than for self-rated health. The results also suggest that union dissolution may be selective of less healthy mothers, whereas union formation does not appear to be selective of healthier mothers.