The science behind IVF is pretty nuts. And it’s easier to understand it when you first get a friendly basic reminder of a normal cycle. So here you are:
Day 1: the body knows it’s not pregnant, and wants to set itself up to be. So it picks one egg from an ovary, and dedicates its resource to growing that one egg. It grows in a follicle: think of a water ballon (the follicle) with a grain of sand (the egg) inside. Over the course of 14 days, the follicle grows in size from microscopic to about 20mm across.
Day 14: the body releases the egg, it travels down the tubes, and splash! lands in the uterus. If it has bumped into a swimmer along the way, the egg is fertilized, and it will implant itself in the lining of the uterus.
Day 28: if not pregnant, shed the lining, and start the whole thing over.
The important part in all this is: one egg inside one follicle from one ovary, on any given cycle.