In this chapter, we investigate what can be termed multi-partner fertility, i.e., the birth rate among women at risk of having a child with a new partner. We used data from 14 European countries and the United States, all with high-quality birth and union histories. We divided a woman’s exposure to the birth risk into three types – single spells up to and including the first birth, spells in a marital or cohabiting union up to and including the end of the first fertile union, and spells after a first non-union birth or after the end of the first fertile union. The last set of spells are those in which a woman is at risk of having a child with more than one partner. Age-specific fertility rates were estimated and combined to generate fertility rates for each union status across five decades in 14 European countries and the United States. We found that, with one exception, multi-partner fertility is quite modest, up to 9% of total fertility. In the United States, however, multi-partner fertility contributes more than 20% of total fertility. Countries with relatively high rates of non-union first births also have relatively high rates of multi-partner fertility. Multi-partner fertility is spread out across older ages, in comparison to single-partner fertility that peaks sharply in the early- to mid-20s. Although the exposure to risk of multi-partner fertility has increased over the decades observed, rates of multi-partner fertility have remained relatively stable.