What is a teratoma tumor?

Since the doctor and the autopsy agree that a teratoma is what caused Kayla to die, I have been trying to figure out what it is and why it happened.

http://en.wikipedia.org/wiki/Sacrococcygeal_teratoma  A bunch of medical sounding stuff.

Teratomas are often featured on those "mystery diagnosis" type of shows because they are this weird tumor that can have different kinds of mass inside of them, like teeth, bones, organs, brain tissue, etc.  Sometimes it almost looks like there was an unformed twin in there.  The autopsy didn't say what, if anything, was inside of Kayla's tumor. Creepy! 

Teratomas form from the beginning of development, from what I understand.  However, her tumor was undetected on my 18-week ultrasound.  If it had been detected, I would have been more closely monitored and there is a small chance they could have done surgery in the womb or induced me early to do surgery on her immediately.  But I still think it is unlikely she would have survived, her tumor was already 13 cm by 17 cm and weighed almost a pound.  My doctor thinks it just started growing really aggressively after 18 weeks.  Since I had no risk factors and no weird symptoms, I wasn't scheduled for another ultrasound until 35 weeks.  Some babies are born with teratomas that are removed and they go on to have normal lives.  But hers was just too big and her heart couldn't keep up.