You have about a 15-20 percent chance of getting pregnant with a single episode of unprotected sex, right at mid cycle, if you are under age 35. At other times of the cycle ovulation is possible, so the general 'fecundity' rate per cycle day is 8%.
Once you have tried for a year, and have not conceived, you are considered infertile. Some of the common causes of infertility can be found here. The government tracks who is fertile and who is not, and you can check their statistics at their site.
Fertility is very age dependent. A much higher percent of women over 40 are infertile than women under 30 of course! Their main trends on site are the following:
Number of women ages 15-44 with impaired fecundity (impaired ability to have children): 6.7 million,
Percent of women ages 15-44 with impaired fecundity: 12.1%
Number of married women ages 15-44 that are infertile (unable to get pregnant for at least 12 consecutive months): 1.5 million
Percent of married women ages 15-44 that are infertile: 6.0%
Number of women ages 15-44 who have ever used infertility services: 7.4 million