Infertility is linked to fecundity. Fecundity for an average 20-ish year old woman who has unprotected sex with her husband in a monogamous relationship "like rabbits" (several times a day) runs at about 30% per menstrual period.
If there is nothing wrong with either partner, she has a 70% chance of not becoming pregnant each month. Assuming there is nothing wrong with the fertility of either partner, working the maths out, over 3 months she has about a 65% chance of becoming pregnant, over 6 months a 88% chance and over 12 months 98%. No surprise then, that fertility clinics will refuse to see a new couple unless they have been trying for at least a year with no success.
it is also said that 1 in 5 couples will experience "some difficulty" in conceiving. that is, they will have tried for at least a year with no success. About 80% of these experiencing trouble will eventually conceive without any medical intervention within the next year or two. Of the remaining 20%, more than half will successfully bear a child with medical help and less than a fifth of these (a fifth of a fifth of a fifth = 1/125) will never be able to have a child without measures such as in-vitro fertilization (IVF).