Each month in a natural menstrual cycle, your body withdraws one gumball (egg) from the machine (your ovaries). That egg is either healthy or unhealthy. As you age, you have fewer gumballs in the machine, and a higher percentage of the gumballs are unhealthy, lowering the chances that the one you get in a given month will be able to result in a healthy baby. That’s why it can take much longer—many more menstrual cycles—for older women to get pregnant.
Even when fertility medicine comes into play, the relationship between age, fertility, and egg quality affect chances of success. Fertility treatments like in vitro fertilization use hormone medications to prompt the ovaries to produce multiple eggs in one cycle, but some of those eggs will still be abnormal (how many are abnormal will depend on your age). So while IVF can help increase the chances of finding some healthy eggs, it can’t make more of your eggs healthy. If a woman in her 40s retrieves 10 eggs in an IVF cycle, it’s likely that only 10–20% of those eggs will be normal. If she retrieves 20 eggs in two IVF cycles, still only 10–20% of those eggs will be normal—but she’ll have more to work with because she retrieved more.
That’s why we see declining success rates as women age, even when using advanced reproductive technology like IVF. If there are very few healthy eggs to work with, the chance of success is low.