There are quite a few different reasons why men or women think they might be unable to have children. You might suffer from a condition such as polycystic ovary syndrome (PCOS), have contracted an STD like chlamydia and PID, pelvic inflammatory disease resulting from it in the past, or have simply been trying to get pregnant for a long time without success. Some women simply have a feeling they are infertile, and sometimes that intuition is proven to be right. Men, too, have varying reasons for thinking they might be infertile.
Whatever your reasons for thinking you might be unable to conceive children, at least naturally, it is a trying and horrible experience that can take a toll on your whole life and can even lead to depression. If you have been trying to conceive actively for a long time (a year for those under 35, and six months for those aged 35 and above), seeking medical help to confirm or deny that you are infertile might well be the right step for you. If you have PCOS, endometriosis, or some other condition that potentially affects fertility, medical checkups are also in order. Won't being fully aware of your medical condition pave the way to either conceiving naturally, or else being able to get treatments such as IVF more quickly?
If you do not have any specific reason for think you might be infertile and want to start trying to conceive, focusing on a healthy lifestyle and diet, and trying to get pregnant naturally, might well mean you soon find out that you were not infertile after all. Infertility is a difficult topic. Thinking you are unable to have children is a truly heart-breaking experience. But thankfully, fertility treatment is becoming more widely available and more effective all the time.