Much like the combined pill, birth control patches contain two hormones. One is a progestogen, and the other an oestrogen. These hormones occur naturally in your body too, and they impact the motions it goes through each month to prepare itself for possible pregnancy. Here are the basics:
Ovulation is when the ovary releases an egg. If that egg is fertilized, it travels to the uterus and attaches itself to the wall to grow. The lining of the uterus (aka the endometrium) gets thicker just before this, making it easier for a fertilized egg to settle in.
The hormones in the birth control patch stay busy trying to block every part of this process. They can stop ovulation, keep the endometrium from growing thicker, and change the density of the fluid in your cervix so it’s hard for sperm to swim through. So no egg, no welcoming walls to attach to, and no fertilization.