I'm 30-years old, but due to my disabilities (AS, anxiety, ADHD, etc), I still rely on my parents for some support, although I now live in my own house. I came out as gay to them 10-years ago, which was the most horrible experience in my life. They are very religious, so they quoted the bible at me, cried, etc. It wasn't bad compared to some, but I think the fact they weren't really supportive of me either hurt a lot because I was having trouble accepting it in myself. They didn't disown me, but they never really mention it, although I know they are still anti-gay marriage.
Being trans, however, is much worse because after my parents had my sister, they lost a baby boy before I was born, and were told they would never have any more children. My father was reading his Bible one night and he felt God "spoke" to him through scripture, telling him they would have another child, I would be a BOY and they had to call me <insert biblical name meaning "gift from God"> (don't want to give away my real name). My father believes this with all his heart, so if I come along and say "um, actually, I'm a girl now and my name is Sarah (or something)" I don't think he would take it very well. I myself am an atheist and believe the doctors were simply wrong (my father later admitted they told my mother it would be "difficult" for them to have children, not "impossible"). I didn't come out unscathed, however, as I have aspergers, depression, anxiety, adhd and now trans on top of everything.
I am terrified for several reasons:
1) My parents will out right reject me and never accept me as female
2) Even if they do accept it, it could shake their religious beliefs to the core (he's believed I was a "miracle" for 30-years). Even though I think they're silly, they're obviously very important to them and I'd rather they died happily believing they were going to heaven, etc.
3) My parents have bouts of stress/depression and I don't want to exacerbate it
4) Even if everything goes well I could still end up killing myself if I can't handle being abused on the street, etc (I also have social anxiety).