Approximately 15% of all couples suffer from infertility at some point during their lives. In most cases, the couple experiences diminished fertility (subfertility), i.e. it takes more than one year to conceive. Sterility denotes complete inability to conceive children, and it is relatively rare. 80-85% of couples engaging in sexual relations without contraception will conceive within one year. Of the remainder, half will conceive without treatment within the following year. In about 25% of cases, infertility is caused by a female factor and in about 25% by a male factor. An infertility-causing factor is identified in both partners in about 25% of cases, and in the remaining 25% the reason for infertility remains unknown.