Like in Among the Hidden, people are forbidden from having kids, although here most people aren't even allowed one, whereas in Among the Hidden families could have two. The governments feared over population with everyone living indefinitely. Thus, only if the parents opt out of taking the drug will they be allowed to reproduce.
What I wonder though is why the rules are quite that strict. Here's the thing. I agree that over-population is a definite concern, but it's not like most people are actually going to live forever. At one point, Malley mentions that crime has essentially been eradicated, because, apparently, it's not worthwhile if you're going to live forever. Really, that seems like bullshit to me. There would still be murder and there would still be accidents. People would be dying off, obviously at a very reduced rate, but there's still no need to institute a "life for a life" policy.
I still haven't really warmed up to Anna, although she definitely improved as the book moved along. At the outset, she was insufferable, with her complete belief in her own worthlessness and her desire to be the best slave ever. Ugh! Plus, as the prefect at Grange Hall, she helped make the already awful lives of other Surpluses even worse. Not exactly a heroine.
Still, I am definitely going to read the next book, as I am curious to find out what will happen next.