IVF usually follows the following process: Ovarian stimulation -> Egg aspiration -> Fertilization -> Embryo transfer. Since ovarian stimulation involves injection of hormones to induce the maturation of multiple ovaries, the concept of ovarian reserve becomes important. This is followed by egg aspiration, where matured eggs ranging from 10-20 in numbers are extracted and subjected to fertilization. Later depending on the quality and growth of the embryo the healthiest ones are transferred back into the uterus.
But when the pool by itself has lower reserve, IVF is severely affected. As, there will be not many eggs available, the process of selection though easy becomes risky and unpredictable.
However studies now have emerged to predict the success rate of IVF depending on the levels of Anti-Mullerian hormone, but there is still a lot of room for research present.