A girl is fertile if she can get pregnant and a guy if he is able to father a child.
The most fertile period for a girl is around ovulation. So if you want to become pregnant it’s best to have sex in between periods (see also: menstrual cycle).
Not everybody is equally fertile. Some couples are pregnant after having sex once, while others have to try for months or even years.
There is diminished fertility if a couple has sex regularly for a year and has not conceived. About 60% of these couples will conceive in the long run, with or without medical help.
Diminished fertility is often caused by problems with the quantity or quality of sperm, with ovulation and/or the condition of the fallopian tubes.