I accedently were in a room were some1 was smoking marijuana, but in a window so the smoke did't really entered room, i.m 20wks pregnant could it harm?

Why didn't you leave: You didn't say how long you stayed in the room? Accidently? It matters most, i think, as to how long the exposure took place. In general-a few seconds of smoke should not really have any significant effect on a developing fetus? Finally, there's really not much you can do about it now, other than avoid similar such situations in the future. Good luck.