What does Pregnancy category B mean?

Pregnancy category is done in pharmacology. There are 5 categories. These are A, B, C, D and X. Category A means there is no possibly adverse effect of a drug on fetus. category B means there in no possible adverse effect on fetus on Laboratory Testing. category C means these is a possible adverse effect on laboratory testing animals. category D means there is a possible adverse effect on laboratory testing animals and on human fetus. And, category X means there is 95% chances of adverse effect on human fetus.