Is human fertility declining?

During the past decades, we have witnessed a remarkable decline in fertility rates (number of births per 1000 women of reproductive age) in the industrialized world. It seems beyond doubt that the enormous social changes of our societies play the major role in this decline, but we argue that reduced fecundity (the ability to conceive) in the population may be a contributing factor. The time taken to conceive (Time To Pregnancy, TTP) has proven a valuable tool in measuring fecundity of a couple, but few studies have attempted to study trends in TTP. Another proposed marker is the spontaneous dizygotic twinning rate, which has declined until the 80s where the use of ovulation-inducing agents introduced increase. Fecundity may be affected both by female and male factors. Semen quality may have deteriorated considerably during the past 50 years and large geographical differences exist. Not many studies among unselected men from the general population exist, but when studying semen quality in an unselected population of young Danish men we found an alarmingly low semen quality. In addition, other biological markers of male fecundity may have changed in prevalence-hypospadias, cryptorchidism and testicular cancer. Changes in ovarian function (biological female factors) may also contribute to a change in the fecundity, although the literature in this field is sparser. We lack information on the roles of modem lifestyle and food contaminants (including endocrine disrupters) on reproductive function in males, as well as in females. It is therefore important that we, even though it is time consuming and costly, continue to monitor semen quality and begin to monitor fecundity, including trends in waiting time to pregnancy. (C) 2004 Published by Elsevier B.V.