What is the main conflict in Bumped by Megan McCafferty?

The novel is set in the near future and a virus has made it unlikely that any woman over eighteen can conceive a child. Melody Mayflower and Harmony Smith are twins who were separated soon after birth. Both girls required medical attention after being born and both were addicted to whatever drugs that their mother was using prior to their birth. The organization overseeing their adoption felt there was a better chance to get the girls adopted if they were separated. Harmony is adopted into a strict religious community and raised there. Melody, meanwhile, is adopted by a well-educated couple who see teen surrogates as the way of the future.