Oestrogen at low levels stimulates the release of eggs. However at high doses, oestrogen will inhibit FSH production. After taking the pill for a while, egg development and release will stop completely. If there is no egg available, there can be no baby. This is why contraceptive pills are so effective.
Oral contraceptives are more than 99% effective if taken correctly and can reduce the risk of certain cancers.
However, there are possible side effects, such as changes in weight, mood and blood pressure due to high levels of oestrogen. Modern pills contain much less oestrogen.
Contraceptive injections, implants or skin patches contain slow release progesterone to inhibit the maturation and release of eggs.