Infertility
A couple is usually considered infertile if pregnancy has not occurred after one year of unprotected, well-timed intercourse. This rule does not apply to couples where the female is over 35 or where either partner has a history of fertility-related problems. If you are a woman over 35 and have been trying unsuccessfully for over 6 months, you should consult your physician. If you a woman over 30 with a history of gynecological problems, or if you know that your partner has a low sperm count, you may not want to wait even that long before seeing a specialist regarding your fertility status.

Primary infertility vs. Secondary infertility
Primary infertility describes couples who have never been able to become pregnant after at least 1 year of unprotected sex (intercourse). Secondary infertility describes couples who have been pregnant at least once, but have not been able to become pregnant again.

Prevalence 
Generally, worldwide it is estimated that one in seven couples have problems conceiving, with the incidence similar in most countries independent of the level of the country's development. Fertility problems affect one in seven couples in the UK. Most couples (about 84 out of every 100) who have regular sexual intercourse (that is, every 2 to 3 days) and who do not use contraception will get pregnant within a year. About 92 out of 100 couples who are trying to get pregnant do so within 2 years.