Idk if I should believe my friend

So I have this friend that has been dating this guy for less than a month and yesterday she supposedly told me she’s pregnant. But she’s only 16 and we’re in our freshman year of Highschool. But I don’t know if I should believe her that she actually is or if she’s just messing with me. She said she’s one week and I told my mom last night what my friend told me, and she said it’s not possible to know if you’re pregnant u til you’re like 4 weeks into your pregnancy. She told me she’s one week so idk. You tell me what you think and how to know if she’s lying or being deadass.