There is strong evidence that the health risks associated with adolescent pregnancy are concentrated among the youngest girls (e.g. those under 16 years). Fertility rates in this age group have not previously been comprehensively estimated and published. By drawing data from 42 large, nationally representative household surveys in low resource countries carried out since 2003 this article presents estimates of age-specific birth rates for girls aged 12-15, and the percentage of girls who give birth at age 15 or younger. From these we estimate that approximately 2.5 million births occur to girls aged under 16 in low resource countries each year. The highest rates are found in Sub-Saharan Africa, where in Chad, Guinea, Mali, Mozambique, Niger and Sierra Leone more than 10% of girls become mothers before they are 16. Strategies to reduce these high levels are vital if we are to alleviate poor reproductive health.