Because you have no official legal status, the medical community may not allow you to authorize medical treatment for your stepchild. And because, legally, stepparents have no authority, care providers have developed some policies to deal with the issue.
Say you walk into Emergency with Billy, who is choking to death on a cherry-flavored popsicle stick. Yes, they'll treat him no matter who brings him in; delay would cause serious damage or death. And if you bring him in with a broken arm, they'll probably treat him at your request, though they may try to delay until they reach a bioparent. They may very well not treat him if he needs major surgery—at least until a bioparent can be located.
I Kid You Not!
You're lucky if you live in Missouri, at least in terms of medical consent laws and stepparenting. There's a statute on the books that empowers parents to consent to a child's medical treatment—and it defines “parent” to include stepparents.
You can get around this system partially (or at least save Billy some pain) if you have your partner grant you power of attorney, or if your partner (and your partner's ex) have signed a form authorizing you to represent your partner when consenting to medical or dental procedures. You may want to carry copies of these forms with you, as well as file them with your pediatrician and your local hospital.
Power of attorney forms and medical consent forms vary in their language from state to state. You can get free blank forms at any local hospital and many doctors' offices. You'll need witnesses.
Following is a simple medical authorization form to be given to the physician in charge. (You can use this for baby-sitters, too.) It puts the physician in charge, and, as such, it's not meant as a final solution. Both bioparents must sign the form (contact an attorney if you can't get agreement from bioparent #2), but it should work for emergency situations and can tide you over until you get the real forms that give you rights to consent: