"So I've never had sex, but I still think I'm pregnant."

It seems your friend is to give birth to the second coming of Christ.

Update: And for the definition of sex, she's never been kissed, and never seen a penis. Or semen, for that matter.
_____________
She believes to be pregnant, has convinced herself she is or something like that but SHE IS NOT PREGNANT.
Period.
Ignorance maybe?
Does she know what procreation is? What gestative functions and pregnancy are? Why she has her period every month?

If she's mentally normal it's maybe:
-an hormonal disorder or
-auto persuasion (but WHY then?The desire to have child, to acquire a certain place/function in the family or society or acquire a personality through motherhood?
(Or is the auto persuasion part of a neurosis? ?)

If she's not it's
-a neurotic behaviour (my guess)
-Or a neurotic behaviour that could lead to psychotic behaviour (extremely rare!)
-Or psychotic : it takes a shrink to tell the difference And he couldn't do it on an internet forum.

pseudocyesis: link
pseudocyesis as neurotic behaviour preceding psychosis: link

-Maybe something like the transverberations of Saint Theresa of Avila? link
Too many maybe's
___ WAY beyond my competences
I once helped a girl through a begin of anorexia over the internet, I received confidences of nearly unbelievable (and unmentionable) things from guys and girls, but I'm no shrink. I don't aspire to be one. No way.

SHE IS NOT PREGNANT.

It CAN be a "false pregancy": link link
This can be very misleading and it is not uncommon among dogs (bitches) of pure race. Among humans it is VERY rare. The last time a virgin had a baby they started a religion about it. ;)

Either:

a) They are not pregnant.

b) They are lying about having sex (or have a very weird definition of what "sex" is).

I've only heard young girls (i.e. teenagers) say things like that. Their problem is usually that they're ignorant about the reproductive process and the way their bodies work. You can't get pregnant if you haven't even been with a guy. If she's skipped a period, there are a number of potential causes. Considering that she's still young, there's a good chance her period is just irregular.

Well, she's either lying about the sex or she skipped her period for normal girl reasons. Ask her why she thinks she's pregnant, and if she's really convinced just go buy a pregnancy test at the drug store over-the-counter, and pee on it.

You can skip a period if you stress or exercixe too much. I notice you're sixteen. You'd be a late bloomer, but some girls do start that late especially if they are skinny. When a girl first starts she isn't regular. Also, as bubbles says, birth control can really mess with a person. And it doesn't help to ask your friends if birth control is making them feel pregnant, because every woman responds differently. That’s why there are a ton of different brands and doses. Also like wanacot said, what do you mean by sex? Things like "pulling out" or other acts which leave semen around the vaginal area can still get you pregnant. Sperm "swim"; they can move on their own. They could travel from just outside, to far enough inside to get the job done. This would be rare, but so much more believable than a "virgin birth." By the way...why do you think you're pregnant?

Wow, first you do need to have sex to get pregnant, no questions asked. Or your lieing about having sex's. but as a teen once myself I never recall feeling like I was pregnant all the time, it could be very well hormones being a teen they are pretty crazy and out of wack. Are you takeing any birthcontorl? They can mess with ya too.