What Causes Delayed Menstrual Periods With Negative HPT?

i want to know about my wife actaully she delieverd a baby in last sept from that time to till now we make sex regularly but from that time to till now she is not in period (mc).

Two or three days ago i made preganancy test himself found result negative. i want to know that why period was delay & suggest some medicine for priod and avoid preganancy.