Introduction
Fertility is the ability to conceive and have children. It involves a complex series of biological events including gamete production, fertilization, implantation, and fetal development necessary to ensure a successful pregnancy and live birth.
A number of known and unknown factors can perturb these events and compromise the chance of natural conception. Depending on the underlying factor, the fertility spectrum can range from subfertility, that is, a delay in achieving conception, to sterility, where the chance of treatmentindependent conception is nil (1).In comparison with other mammals, human reproduction is relatively inefficient—as evidenced by the low monthly fecundity rate (the probability of pregnancy in each menstrual cycle). In the absence of any underlying cause for subfertility, it is estimated that around one in five women will become pregnant each cycle, with conception occurring in 84% and 92% over a period of 12 and 24 months, respectively (2). However, the probability of conceiving each month varies significantly among couples and declines with increasing duration of non-conception. These complexities make the task of defining infertility difficult (3) and have led to different ways of describing the condition by clinicians, epidemiologists, and demographers.