This longitudinal study examined the effect of the birth of the 1st child on relationship functioning using data from 218 couples (436 individuals) over the course of the first 8 years of marriage. Compared with prebirth levels and trajectories, parents showed sudden deterioration following birth on observed and self-reported measures of positive and negative aspects of relationship functioning. The deterioration in these variables was small to medium in size and tended to persist throughout the remaining years of the study. Mothers and fathers showed similar amounts of change after birth. The amount of postbirth deterioration in relationship functioning varied systematically by several characteristics of the individual, the marriage, and the pregnancy itself. In a group of couples who did not have children, results indicated more gradual deterioration in relationship functioning during the first 8 years of marriage without the sudden changes seen in parents, suggesting that the results seen in the parent sample may be due to birth.