Many young kids are fascinated by babies and pregnancy. A lot of them will, eventually, ask "how the baby got in there" if their mother is pregnant. That would leave the typical parent feeling slightly uncomfortable and with the dilemma whether to give the details of human reproduction, or tell a "cute story" instead. For parents who feel uncomfortable talking about sex with their young children, explaining fertility treatments may actually be easier than explaining natural conception. You can explain your fertility treatment in age-appropriate terms, saying that your doctor is helping you to get pregnant with the child's baby brother or sister.
You can explain the actual fertility treatment you will have in more detail if your child asks for more information, or just leave it at that if your kid seems satisfied with the answer. You may want to explain that mommy's egg and daddy's seed come together in a small dish if you are having IVF, for instance, or say that the fertility clinic will help daddy's seed get to the egg if you are having intrauterine insemination. If you are comfortable with it, it may be useful to explain natural conception at the same time, and to point out that women can get pregnant in different ways. Some get pregnant naturally, while others need a little more help.
You could even take your child to your fertility clinic once, during the monitoring stage of IVF, for example. That way, what happens at a fertility clinic will become clearer for the child.