Infertility primarily refers to the biological inability of a person to contribute to conception. Infertility may also refer to the state of a woman who is unable to carry a pregnancy to full term. There are many biological causes of infertility, some which may be bypassed with medical intervention.^[1]
Women who are fertile experience a natural period of fertility before and during ovulation, and they are naturally infertile during the rest of the menstrual cycle. Fertility awareness methods are used to discern when these changes occur by tracking changes in cervical mucus or basal body temperature.

The couple has not conceived after months of contraceptive-free intercourse if the female is under the age of 34. There are also similar terms, e.g. subfertility for a more benign condition and fecundity for the natural improbability to conceive after 12 months of trying. Infertility in a couple can be due to either the woman or the man, not necessarily both.

Couple that has tried unsuccessfully to have a child for a year or more is said to be subfertile meaning less fertile than a typical couple. The couple's fecundability rate is approximately 3-5%. Many of its causes are the same as those of infertility. Such causes could be endometriosis or polycystic ovarian syndrome.

Data from UK, 2009.^[9]