Making the diagnosis of infertility is based on the rationale that the chances of achieving a pregnancy are a function of time; the longer a couple tries, the more likely they are to eventually conceive, and intervening too early may lead to unnecessary, and expensive, investigations. On the other hand, there is a cut-off period after which the chances become slim even with further sexual exposure.
Around 50% of couples would be expected to conceive by 6 months and around 80% after 1 year. Since only a minority of couples would not have conceived after 12 months of trying, 12 months is chosen as the cut-off period for making the diagnosis of infertility.
In patients with obvious causes for infertility such as infrequent periods (indicating a problem with ovulation), it is not necessary to wait 12 months before undertaking infertility investigations or commencing treatment. A very important group for whom unnecessary delay could be disastrous is women above 36 years who are entering a phase of accelerated fertility decline. In these women it is justifiable to investigate after 6 months of trying.