I wouldn't be so quick to say that the teens aren't interested. Just because they don't act interested doesn't mean they aren't. Teens-especially pregnant teens--get asumed upon a lot. People assume that they know very little (especially those who were 'dumb enough to get pregnant'). I'm not saying you think that, but doctors and nurses and teachers and relatives often talk down to them, and many are sick of it.
So when you start talking about stages and phases of labor, an "uh huh" or "yep" may be the teen trying to show you that she doesn't need you to teach her. She doesn't want you to know that she's clueless about XYZ, because then she'll be proving everyone right.
Don't assume that she knows nothing-she may know more about swaddling and feeding a baby than you do. On the same account, don't assume that she has any idea what a "contraction" is. Talk to her for a little while to get a feel for her learning abilities and previous knowledge is.
They're also lectured a lot--by teachers in school, by their parents, by their friends.
You're 25....think back to when you were a teen--what could have gotten you interested in this information? I find my teens (and many adult pts/students) respond best to visuals with large colorful pictures, and hands on models to play with as opposed to "This is XYZ, and ABC will happen, so expect 123"
Control issues may be at work as well-maybe their mother picked their OB/GYN and books the appointments for her. Maybe her partner didn't "let" her get an abortion. Her doctor tells her which hospital to go to and what tests to get. Her sister tells her what to eat and when to take her prenatals. Her teachers tell her to do her homework and write a paper on XYZ topic. She feels like she has no control in her life, this pregnancy included.
If you have more than one prenatal visit with these teens, ask them-"What would you like to cover today?". if you only have one prenatal visit, how about questions like "What are you most concerned about?" and "What are you interested in?" That way the ball is in her court.
Also, examine your feelings towards teenage pregnancy and motherhood. Be honest (with yourself, I'm not asking you to tell me)-do you find it morally repugnant? Do you think it's a mark of being low-class? Do you automatically assume that their parents failed? Do you think the teen and her baby are doomed to a life of poverty? Do you assume she'll be a bad parent or that she'll "dump" the baby on her parents?
People can pick up on other's feelings. Even if you think you're acting non-judgemental, you could be sending out vibes that she's picking up.
I'm having a little bit of trouble articulating what I really mean here....if you need me to clarify something, let me know.