Evidence that declining human fecundity contributes to the fertility crisis.

Fertility rates have fallen to unprecedentedly low levels worldwide, reflecting a widespread fertility crisis. Although this decline has often been attributed to social and economic shifts, growing evidence suggests that biological factors may also play a role. Markers of decreased fecundity are on the rise, with an increasing number of couples requiring medically assisted reproduction because of poor semen quality and/or female infertility. In addition, the number of children born after assisted reproduction is rising. Trends in pregnancies terminated by an induced abortion may also provide important information regarding fecundity. A new index, the comprehensive unassisted pregnancy rate (defined by live births and induced abortions, excluding births as a result of artificial reproductive techniques), has been developed to reflect the fecundity of a population. Recent data from a few high-income countries have shown that the comprehensive unassisted pregnancy rate has been steadily declining across successive decades. If this finding is replicated in other countries, it could indicate that patterns of changing fecundity may be emerging as a population-wide international phenomenon.