The patch sticks to your skin and releases hormones (estrogen and progestogen) which are similar to the hormones naturally produced.  A new patch is required each week for three weeks out of four, the fourth week is patch free.

The main way the patch works is to stop the ovaries from releasing an egg each month (ovulation).  It also thickens the mucus in the cervix preventing sperm reaching the egg and thins the lining of the uterus so it is less likely to accept a fertilised egg.