There are substantial differences in fecundability among nine developing countries in different parts of the world. In addition, within countries, later age at marriage has a clear, positive, nonlinear effect on fecundability. Women with higher education and those with more educated husbands have higher fecundability. Urban residents have higher fecundability than rural. The higher fecundability of more recent cohorts is the most consistent observation. Since those variables most frequently shown to have inverse relationships to fertility show direct relationships to fecundability within populations, the role offecundability as a proximate variable in models of the determinants of fertility requires further specification.