U.S. Protestants are less likely to belong to “mainline” denominations and more likely to belong to “conservative” ones than used to be the case. Evidence from the General Social Survey indicates that higher fertility and earlier childbearing among women from conservative denominations explains 76% of the observed trend for cohorts born between 1903 and 1973: conservative denominations have grown their own. Mainline decline would have slowed in recent cohorts, but a drop‐off in conversions from conservative to mainline denominations prolonged the decline. A recent rise in apostasy added a few percentage points to mainline decline. Conversions from mainline to conservative denominations have not changed, so they played no role in the restructuring.