After the egg is fertilized your hormones go mad anywhere but there is one particular hormone – hcG, that the little stick is looking for. If it finds that – boom! You have an embryo and the test tells you so!
There’s a hormone called human chorionic gonadotropin – a mouthful I know, that comes out of the placenta when the egg has been fertilized. The tests specifically look for this and how much of this is in your urine, so they can detect if you are pregnant.