Hi my last period was on dec 21,2007 we have been trying to have a baby from then on. Can you tell me if I were to get pregnant when will my test show it...

The best time to test is after you miss a period.

Understand that you have not even been trying for a month yet, so I don't want you to stress out or get disappointed if you test negative for several months. It can take a healthy couple up to a year of actively trying before conceiving.