For a woman who is under 35, a couple is considered infertile if the woman and her husband have had regular unprotected sex for 12 months without conceiving a child. For women over 35 years old, the couple is considered infertile if they have not conceived a child after 6 months of unprotected intercourse or longer.
Generally speaking, a woman has about 20% of chance of conceiving a child after 1 month of unprotected sex, this chance will be increased to 40% for three months, about 60% for half an year and 80% to 90% after 1 year. Hence, a couple should consider a thorough medical checkup after trying for 1 year if the wife fails to conceive.