IF FERTILIZATION ONLY TAKES UNDER A WEEK. THEN WHY CAN'T WE TAKE PREGNANCY TEST UNTIL WEEKS AND WEEKS LATER?

SO EVERYONE SAID FERTILIZATION(WHEN SPERM REACHES THE EGG AND GET PREGNANT) ONLY TAKES UNDER A WEEK. SO HOW COME WE CAN'T TAKE PREGNANCY TEST UNTIL WEEKS AND WEEKS AND WEEKS LATER?