How does a couple know if they are infertile? To understand infertility, it is very helpful to understand fertility…normal fertility. When or how quickly conception will occurs has been well documented.
A couple with normal fertility will have an average rate of conception of about 15% per month. In other words, 15 out of 100 couples trying for one month will achieve pregnancy. Most couples will conceive in the first few months, in fact if fertility is normal, 70-80% of couples should be pregnant after only 6 months, and the chance decreases per month thereafter.
Immediately, all sorts of other “what if’s” come to mind, such as a woman’s age, are the periods regular, was intercourse at the “right” time, are tubes open, and is semen quality normal. But since we are talking here about an “average” among couples with “normal” fertility, all of these are not issues here. Looking only at 20-year-olds, they would conceive faster. Women after 35 years of age conceive more slowly than the average.
Although the percentage conceiving per month will decline with age, this same information suggests that 85% of normal couples should be pregnant at the end of 1 year. If the couple has not conceived in 1 year, or for women over 35 within 6 months, it is much less likely to happen in the months beyond this time frame. This is why the definition of infertility is: unsuccessful pregnancy within 1 year. This understanding is also why an evaluation is warranted after 1 year of trying to conceive.
If a woman or couple already knows of factors that would affect conception rates, there is no need to wait a year to be evaluated. Problems like absent or irregular menstrual periods, PCOS, obesity, smoking, prior gynecologic surgeries, and male health issues are reasons to seek an evaluation earlier.
Although the percentage conceiving per month will decline with age, this same information suggests that 85% of normal couples should be pregnant at the end of 1 year. If the couple has not conceived in 1 year, or for women over 35 within 6 months, it is much less likely to happen in the months beyond this time frame. This is why the definition of infertility is: unsuccessful pregnancy within 1 year. This understanding is also why an evaluation is warranted after 1 year of trying to conceive.
If a woman or couple already knows of factors that would affect conception rates, there is no need to wait a year to be evaluated. Problems like absent or irregular menstrual periods, PCOS, obesity, smoking, prior gynecologic surgeries, and male health issues are reasons to seek an evaluation earlier. 
