It is a birth control and is not intended for use in early pregnancy. Another name for this drug
is Provera (popular birth control). It
is a category X drug in pregnancy, meaning it can cause permanent damage to a developing embryo. I don't know how long it can stay in your system so that is probably why you cannot take it in early pregnancy.
Next, call the pharmacy and speak with a pharmacist. Let a professional explain to you why you should not be taking this drug while pregnant.