The development of female gametes (eggs – oocytes) begins in the prenatal period. The vast majority of the original 2-3 million primary oocytes perish and only 300 thousand remain until puberty. Only about 450 eggs are released during ovulation in the female fertile period.
Normally, each month there are about 25 follicles (in which eggs grow)  prepared in the ovary, but only one egg  matures and is released. For the IVF method it is necessary that a higher amount of eggs are prepared in the ovaries an therefore, so-called, ovarian stimulation is induced by hormonal medication.
A higher amount of follicles (cavities growing in the ovaries) develop after administering the hormones (gonadtropins).
The growing follicle produces estrogen, which triggers the rise of a luteinizing hormone (LH). In a natural cycle it allows the release of mature eggs from the ovary. During the stimulation, a fertility drug, that delays natural ovulation and prevents a premature LH rise, is administered.
Ovulation (egg maturation) is induced by injected chorionic gonadotropin (hCG), which must be  applied about 32-36 hours before the scheduled egg retrieval.
The doctor stipulates the beginning, duration and method of stimulation according to a woman’s individual reaction, her age and weight.
The doctor also monitors the organism’s reaction to ovarian stimulation. The ultrasound is repeatedly used and the hormone level in the blood is inspected. The main reason for these inspections is to induce an optimal ovarian response.
On one hand, it is important to reach a reasonable number of eggs, on the other hand, it is an attempt to prevent an excessive reaction (hyper reaction) manifested by pain, a distinct enlargement of the ovaries and the formation of free fluid in the abdominal cavity (OHSS, ovarian hyperstimulation syndrome). The hyper reaction is most dangerous for women under 30.  
Testing has no side effects (neither repeated testing) on eggs or embryos in the early stages of pregnancy.
Administration of hormonal drugs does not exhaust the supply of eggs in a woman’s ovaries, it only prevents the natural degeneration and the selection of the dominant follicle. According to current research, it does not have any further adverse effects on the body.