The Human egg is very small and grows within a small tiny cyst called follicle. Each month at the beginning of the cycle one follicle grows from a size of 5mm, to a mature size of 20mm on the 13th or 14th day of the cycle, when it ruptures to release its egg.
It is now possible to follow up the growth of the follicle by ultrasound, this is called follicle tracking. It’s also possible to release the egg 36-40 hours late by means of an injection (HCG). Follicle tracking is usually carried out in assisted conception cycles in order to time intercourse, or carried out in conjunction with artificial insemination.