My Name Is Tunde. My Question Is That My Wife

My name is Tunde. My question is that my wife av four week pregnant and Doc describe Gestin 5 mg, twice in daily after take gestin for two day she see sin of black blood in private body and the blood stop she reset another pregnant test and the result is till a postive.So my question is that what cause this blood because the blood is still coming yesterday and am totally confused.