There are four ways the pill acts to stop sperm reaching an egg (ovum). First, the hormones in the pill try to stop an ovum being released from your ovary each month. This is known as the suppression of ovulation. Research has shown that neither the progesterone-only pill nor the combined progesterone-oestrogen formulations always stop ovulation.
Second, all formulations of the pill cause changes to the cervical mucus that your body produces. The cervical mucus may become thicker and more difficult for sperm to fertilize an ovum.
Third, all formulations of the pill cause changes to the lining of the womb (properly known as the endometrium). Under the influence of the chemicals in the pill, the lining of the womb doesn’t grow to the proper thickness. You will notice that your periods are lighter when you are on the pill. This is because the lining of the womb has not developed properly. But this change also means that the womb is not in the right stage of development to allow a fertilized egg to attach properly (this attachment process is known as implantation). This action of the pill will be discussed again in this booklet.
Fourth, the pill causes changes to the movement of the Fallopian tubes. This effect may reduce the possibility of the ovum being fertilised.