Scarleteen

Ok so I want to get pregnant but I haven't had my period in a while now so how to I find out why ? I'm 18 and my boyfriend and I want a child together

Have you already gone to an OB/GYN? If not, that's always the best thing to do before trying to get pregnant anyway.

That appointment can make sure you're in sound health for a pregnancy -- and help figure out what's up with your periods, and the gynecologist/obstetrician can also give you information about how to start charting your fertility and timing intercourse with what that charting tells you to make conception more likely.

I have they told me they don't know why

The doctor you saw: did they do an exam, run any tests, or refer you to another doctor who might be better able to figure out what's going on? If not, then it's time to get a second opinion. Missing periods are really something only a doctor is going to be able to help you with, and if the one you saw wasn't helpful, then seeing someone else is the next step.